Security Problem?

Well you should have a disk cleanup, it should be somewhere in the start menu in system tools. However Norton has been known to be a huge computer lag, it slows it down big time. I no longer use Norton, nor does my family, we have since swithced to AVG and find it to work much better, it doesn't slow our computer down and protects them just as good, no programs at all.

try CCleaner from www.filehippo.combut i don't think it is as much of a security issue than it is an OS issue. Vista has been known to reduce performance due to its numerous processes it runs at the same timebut give ccleaner a try.