lavasoft ad-aware+spybot s&d to start, and there are good things being said abt combofix & malwarebytes. if you want to spring for a commercial anti-virus, nod32 is AWESOME.
google chrome for fast browsing, firefox for highly customizable browsing (with the necessary protective extensions like adblock and such), google chrome frame for ie for those times you’re forced to use ie (windows updates if you don’t have automatic updates turned on).
cccp codec pack like d3v said, realalternative+tara for realmedia playback in winamp, vlc player for any remaining video format. i use winamp for audio (VERY extensible, and in some cases the ONLY way to listen to some formats), but others will swear by foobar2000 (i avoided it for a long time cuz it wasn’t updated).
replace the default notepad w/either notepad2, notepad++, or programmers notepad. if you choose to program, there are also a whole lot more choices on windows than mac.
paint.net to replace the default paint, gimp if you want a free photoshop-like. mac photoshop always was better than win photoshop, tho, and mac gimp is…kinda gimped.
being a windows user who converted to mac long ago, it really is kinda hard to replicate the mac experience on windows, but windows 7 seems to go a long way towards that, and in some cases (particularly w/games) it’s just as hard (if not harder) to replicate the windows experience on mac (like system sound schemes–that’s built-in on windows, but mac users need a “haxie” that doesn’t even work on snow leopard).