sara and hilly are right
one other thing though how much free discspace?
windows and the ntfs file system likes 20% free to work in
that means you need 30%+ free on your windows partition
for cleanup
http://www.ccleaner.com/download
(it is free - but asks for a donation)
run with defaults - if you bank and log onto sites you might want to leave cookies intact
also if you have a huge amount of small files - mp3 or jpgs
that can affect performance simply because the contents database is huge
1x4Gb file needs only 1 MFT entry
4Gb of JPGs could need 10000s of entries to search