OS X :: IMac Very Slow After A Few Hours Use - Especially When Using Photoshop

I have a fairly new iMac (mid-2008 20" model) with a 2.4ghz Intel processor. I am a web designer and use this machine every day for my work. Several months after installing Adobe Creative Suite 3, I noticed that Photoshop was causing the cursor to jump and/or become jerky, as if the video card or processor were not keeping up with what's going on. This tends to happen after the computer's been in use for several hours or days after a restart. I have been following this in Leopard's Activity Monitor, and I noticed that Photoshop (being memory- and processor- hungry) taxes the processor more heavily than many other programs, which I expected. The strange thing, though, is that over time the Activity Monitor shows the "System" as taking up more and more processor capacity. I.E. when I first start up, the system is using 0-20% of the processor, the rest being available for the user; but after awhile, the system is requiring 50-90% of processor capacity, the result being that as soon as I switch into Photoshop from another program, the system is immediately overtaxed and starts moving very slowly (especially the cursor, as I said).

Now, here's the really strange part. As I said, this began happening a few months after I first installed Adobe CS3. I re-installed the software at Adobe's recommendation, and it was fine for another few months, but it began happening a few days ago, which is exactly the same time I shared a jump drive with a friend's worm-infected Windows XP machine. I know Macs don't get Windows worms, but is it possible that this thing has lodged itself on my system somehow and is causing it to run generally more slowly?