disclaimer: you might need a slightly nerdish attitude to enjoy this post.

especially as a windows xp user you lack some basic tools when you've freshly installed your os out-of-the-box.
personally i had some envious glances at famous linux distributions which are way more generous.
depending on whom you socialise with you will most likely get tips here and there on some shareware or good commercial apps.
which you might end up continuing to use way beyond the deadline set by the license (if the application allows it)
or maybe you even take a peek at the 'darker realms of the net' to unlock it.

however, most purposes can be served more than satisfactorily by freeware, and often open-source too, apps.

it starts with the most basic thing. a nice small text editor.
f'ing notepad.exe doesn't even tell you which line you're on.
i suggest using notepad plus . works great, does even display those
.nfo files you may occassionally stumble upon the way it's meant to and is also capable of advanced stuff like syntax highlighting for tons of programming languages. you can even collapse
statements like 'if', 'while' to get a quick overview over your code.

for more sophisticated office works openoffice is all you need really.
i suggest to try the 2.0 betas.
openoffice also allows for direct pdf-export which is a huge advantage over ms office.
who can afford adobe distiller or some commercial plugin really, just to release some paper?
actually being able to export to pdf is something you get so used to,
you will eventually want to be able to do it from all applications.
which is where pdfcreator comes in.
once installed, you have one additional printer in your system, namely pdfcreator, and use any print dialogue to create a pdf.
you will need to download a larger package with ghostscript if you don't have that already.

let's continue with graphics. so microsoft offers the marvellous paint.exe.
fantastic, innit?
well, for some serious drawing or photo editing you have to use something else, no doubt.
the gimp is a powerful albeit not too intuitive tool from the linux world.
you will also need to install gtk (provided there).
recently there's been an alternative compile. the menus there resemble the adobe photoshop layout more.
hence gimpshop. note that gimpshop is not maintained as frequently as the gimp.
a bit more photo-orientated and userfriendly imo is paint.net.
that's actually something being developed from some fresh young microsoft
employees as a paint replacement and matches those $(1)99 commercial apps!
if you need to get some vector graphics done (think corel draw, adobe illustrator) then there's no better freeware alternative than inkscape.
it's beyond words really.

i guess one of the big differences between users is how they navigate through their files in xp.
i've seen some who stack their desktop with folders and files they need all the time.
besides the fact that i like my desktop clean and slick and not stuffed you should know that when your windows gets really fucked up and needs a reinstall it's likely that your document & settings branch, including the desktop and 'my files' will be gone.
then there's the people who use the ms explorer. however the lack of a second pane makes it hard to copy & move files quickly and it's less convenient than the old norton commander really.
personally i belonged to the third kind, i just navigated using lots of windows starting from shortcuts to all my drives.
drap & drop between windows to copy, etc. but it's kinda tiresome to branch deep into trees.
recently i changed to a 2-pane explorer replacement. some friends of mine
've been using midnight commander, the gnu open source alternative.
the old dos-look never appealed to me though. so i stick with xplorer2 lite
it's sort of the only commercial app here, since there is a more powerful version on that site too that has to be payed for.
the free version however is powerful enough, and after a few days i don't wanna live without it.
bookmarks for folders are handy

most downloads so far come as zip files. windows has some more or less comfortable zip support, but there are way more formats, and some more comfortable ways to handle them anyway.
lots of shareware out there, sure. winzip, winrar, powerarchiver...
but the open source freeware alternative is 7-zip. it extracts most of the formats, except ace. and can create enough.
the new beta finally supports drag&drop for its main window, there's also a command line version
all you need.

audacity suits all your needs for wave editing. you can use it as a multi-track sequencer too.

you might have seen sfv or/and md5 files.
those are checksums, file fingerprints.
recalculating them after download and checking against these files is the way
to assure file integrity.
i'd like to point out that it's vital to use them with open source tools,
especially if you got the installer from a friend or didn't download from the manufacturers homepage.
anybody can recompile and spread own versions. including spy- and malware if that's the goal.
so if you download from some other source you might at least try to get the
checksum from the original homepage and check if the file is the same.
there's this one shareware tool that pops up all over google, but there's powerful freeware too.
try md5summer and sfvmanager

phew. i'm out of fuel. not toolwise, mind you, there'd be some more to mention. but i grow weary ;)
here's a short last one, not much to say, maybe more some other time.
ftp server/client: filezilla