Virus or What?

my dell laptop with vista has recently begun working poorly. the email and
internet now take 60-90 seconds to respond to open, the computer seems to
freeze often, and the mcafee security center seems to turn off (by itself)
and then has to "fix" the "computer and files" and "email and im" portions.

i have tried to return to an earlier registry. i've done all the scans,
reinstalled mcafee 3 times, and still the problems persist. it's like my
computer has been infected by something slowing it down, stopping it from
responding to commands promptly, and occasionally interfering with the mcafee
operation (since i have to keep fixing the security center). i did the
mcafee virtual tech and fixed all the problems it identified. i've updated
all the downloads for windows and mcafee.
i've run out of ideas. i have no clue what the problem is or how to fix it.

Has anyone had this problem? Can anyone help?