CS3 Sluggish

My wife is a currently a heavy photoshop CS3 user in her home office. We recently upgraded to CS3 and I have been chasing performance issues for weeks. We would describe performance as becoming sluggish at times. Sometimes she can go away from the computer, and come back and the problem will seem to clear.

I have tried many things and am not sure where to turn at this point. Her machine is a fairly new Dell Optiplex with an Intel Core 2 processor, 4 GB ram with memory currently set to default boot.ini settings. Hard drive paging is set initial 2 GB max4 GB.
The OS is XP home edition.
I have gone to a lot of effort to go through all software (BIOS, drivers, OS, Photoshop etc…) and rev. up to current versions.

My wife leaves her computer on all the time, and leaves photoshop running for long periods of time without closing or rebooting.
She likes to open a group of photos, around 1-12 (6 mega pixels each), and she will process them and close them as she finishes each one.
Interestingly, with our last Machine, a Dell Dimension P4 with XP Home, 1.5 GB RAM and Photoshop CS, she had no issues with this usage pattern.

This morning her photoshop was running with around 15 photo images (6 Mega Pixels each) open, and performance was a little sluggish. When she is having troubles it is usually worse. I closed all of the images, and noticed via task manager that memory never released (until I finally closed photoshop). At closing task manager was showing a usage around 1.88 GB which was where it was when I started closing images.