Good anti-virus program and a solid firewall is needed to keep the shit out.

Does it start alot of programs at startup?
Using alot of auto-update crap?
Have alot of programs you dont realy use and/or know what they do?

If it's realy bothering you how slow it is, my guess is that it's more then what a simple defrag can fix. Hard to tell from here tho without knowing anything about your comp. No reads on villan!

/format is the safe bet.