I run a windows XP (SP2) desktop, it is about 2 years old.

Intel(R) Core(TM)2 CPU 4400 @ 2.00GHz

So instead of reinstalling my windows, I used repair which just restores the system. After repair, my pc runs very fast, but after using it for a few days, it starts gradually slowing down until it reaches a point where even typing or moving my mouse is laggy. However, this only happens when im multitasking on my windows. If i end all the big processes (antivirus, downloading programs, MSN, etc.), then my computer runs normal again. Another thing i’d like to mention is when im watching a youtube video and I try to open a program, it lags insanely bad for a short amount of time. When I watch a video, even if its loaded, it still lags. I do not use my PC 24/7 but sometimes I leave it on, for downloads. My CPU usage is not necessarily high when my computer is lagging so its probably not a RAM problem. I also have over 50% of free disk space on all local disks.

My PC does not have any viruses, I use Rogers Online Protection, and i am not experiencing any other difficulties except for multitasking slowdown. I’ve tried many things already, I’ve tried to cancel windows updates, stop using anti-virus, I’ve checked all my running processes (searched them on internet) and they were fine, I’ve recorded all the running processes of when my pc was slow and i’ve checked them after repairing my windows. None of these solutions worked. Is this happening to anybody else? Is my PC too old? Could it be a hidden virus? Should I stop downloading and see what happens? Is this a case of software or hardware?

I don’t want to use my computer without being able to download and use the internet at the same time, or cancel antivirus. And I don’t want to repair it every few weeks/days. I am not a pro at these kind of things, so plzz help!