Security:
None of them are perfect...just try to pick the lesser of the evils. Internet security suites from McAfee ($50) and Norton ($50) are both huge resource hogs. Panda ($80) will not let you uninstall it. TrendMicro ($50) has been the best to me, with ZoneAlarm ($60) being the runner-up. I'd recommend downloading free trials and see which suits your computer best.

Office type:
Of course Microsoft Office 2003 is at the top of the list, but a good free alternative is OpenOffice. It is compatible with MS Office, sports a nice interface, offers advanced options, and is currently approaching its 2.0 release. Oh, and did I mention it is free?

Free System Maintainance:
CCleaner: Helps clean out old and useless files on your HD...not just the TEMP folders.
RegCleaner: Cleans old and corrupted entries from your registry.
Microsoft Antispyware: The best protection from spyware currently available. (Beats SpyBot and Adaware)

Free Misc. Programs
NotesHolder: Great little note-taker.
CursorXP: Customizable cursor sets.
BootSkin: Small utility for changing your boot screen.
LogonStudio: Small utility for changing your logon screen.
Konfabulator: Best free utility for quickly adding widgets to your desktop.
ExtractNow: Small unzipping utility for compressed files with wierd extentions.
Firefox: Best alternative to InternetExplorer.
Trillian: Replaces AIM, MSN, Yahoo!, and ICQ IMs.

I've got a list of over 150 great free programs, but these are the best and most useful.

Hope this helps,
John