Very likely you don't have enough RAM, PhotoShop gobbles RAM like a child in a candy store. How much RAM is on the two PCs you mention?

Have you defragmented your hard disk lately? If not, do so. Your swap file [virtual memory on the hard disk] may not be able to find a big enough block of free space to operate properly.

Btw, what OS have you, and how much free space is available on your hard disk?