Just a couple things I do:
1) Make a list of the porgrams you've downloaded then just download them again if you've got a decent connection. That way you've got the latest updates and it's a clean install.
2) You can just save your Firefox profile if you've got tons of bookmarks and settings saved (like I do
). Clear your cache and history then find the following folder: C:\Documents and Settings\XXX\Application Data\Mozilla\Firefox\Profiles
where XXX is your user name in Windows and copy the profile folder that you use.
As for a firewall I just use ZoneAlarm (free version) and it seems to work alright. And as for antivirus I use Norton and have had pretty good luck with it.