if it's vista the defragger is built in.
VHG and Twenty are pretty much right ... it won't make a huge difference ..... unless your disc is getting full
(ntfs likes at least 20% free to do it's thing)
also if the temp folder is ridiculously full( <1000 files)
this will slow things down.
use CCCleaner to have a bit of a cleanup
http://www.ccleaner.com/download
then if still no improvement .... consider a memory upgrade (come back with the processor, disc details (or just how old is the machine!) - for advice on how much - or visit
http://crucial.com/
or
http://www.kingston.com/