i just reinstalled XP a week ago, so i'm in a similar situation.
Somewhat in order of importance, all free:
Sygate Personal Firewall
Firefox and Thunderbird (or if you want integrated web/email/news/IRC, Mozilla)
Spybot Search & Destroy and Ad-Aware
Winamp (iTunes is good for easily tagging files, but beware of the "keep music organized" feature)
Zoomplayer for video (more streamlined than Windows Media Player
Irfanview for viewing damn near any image format
Zipgenius if you need compression support beyond ZIP
Deepburner and/or Burnatonce for CD/DVD burning (not sure if BAO does DVDs)
Filezilla (if you need a good FTP program)
Open Office is a great office suite comparable to MS Office. You will need to spend some time learning the interface though. If you don't want to do that and still want a legal office solution, drop by a computer show or check online for Office 97. It's got the best feature to bloat ratio of all the MS Offices, and can be had for roughly $35. i'm still looking for a good freeware word processor that is compatible with Office and more open formats.
Bitcomet is my current Bittorrent program at the moment. Nice interface, and the upload/download speed limiting is extremly accurate. Only downside is that you can't set limits to exactly how much you seed.
Anti-Vir and AVG are two freeware virus apps. But i haven't had a virus since '96, so i'm not too worried about reinstalling them now.
Zone Alarm and Norton Antivirus are off of my list of recommended apps due to serious problems using them. ZA killed my internet access completely, and NA had a nasty uninstall the left files (one loaded with Windows) and when i deleted the folder, it killed my internet connection as well. Had to edit the registry to complete delete it.
Exact Audio Copy is my CD ripper. Tons of features and can rip and encode to MP3 quite easily. If you need a good format to rip to and can spare the hard drive space, check out FLAC. Lossless compression for true CD-quality sound, though filesizes are roughly 5MB per minute of audio.
i'd also recommend setting a system restore point immediately.