Defrag might help a bit, but a thorough disk clean up is likely to do more 
noticable good.   The diskcleanup utility from within windows isn't that 
good.  It leaves big things behind that are garbage.
Try ccleaner available at http://www.savemybutt.com
You can also get addaware and spybot there.  Their is also a link to the 
free avg anti-virus program.