By it's very nature, SheetCam is a bit of a memory hog, especially if you have a job with lots of pockets open. Having said that a reasonable spec computer should not have any troubles.
How much RAM has the computer got? For Win2000 you need an absolute minimum of 256Mbytes and for XP, 512Mbytes. Any less than that and you are going to struggle. I would recommend at least 512M(win2000) or 1Gbyte(XP/Vista) for good performance.
Is your hard drive full? If the hard drive is full, Windows may not be able to create a big enough swap file. Try defragmenting the drive. This doesn't increase space but it may make it easier for Windows to allocate a big swap file. Have you got your swap file(virtual memory) set to too small? Normally Windows works out the size automatically but you can override it. I would recommend leaving it on automatic.
Another(remote) possibility is that your display driver is fouling up with SheetCam's graphics. I haven't heard of a driver causing your problem but I have quite often come across drivers that do other weird things. As a last resort, try turning your graphics acceleration right down and see if that makes any difference.