South beach has a point,
slow performance could be a number of things, virus, malware, bad memory, heat...
I was only responding to the PSU part of the question...
Run antivirus scan and malwarebytes confirm no virus / malware...
First thing I normally do, is clean out the case...
then I re-seat my memory, video cards, check connections, etc...
run memtest if suspect... / alternately check with known good memory if available
Then I punt "re-format" - eliminate any unnecessary programs which tend to build up on general purpose systems after awhile... only loading my primary programs... Charting, Office, firefox, Anti-virus, Sandiebox...
I keep my Data on separate drives, only Operating system and programs go on C drive.... before you do anything back up your data if not on separate drives...
if system still slow, most likely hardware related...