defrag is ok to run , but first you may try to get all the ad ware and eventually spy ware out of your machine. a register scan and clean is smart too. try this exellent freeware prog at http://www.downloads.com
ADaware (removes ad ware)
Spybot search and destroy (removes Spyware)
these 2 programs removes plenty off stuff that slows your computer, that antivirus may not discover or take as a virus.
A registry clean program can also be found at downloads.com but they often only repairs first "half" of the problems found and you have to buy program to get the rest.
Summary: 2 important programs to have is ADaware and Spybot search and destroy. These are freeware