A couple things with photoshop that might lock you up. But we need to know what your system is first.
OS? I had a lot of lockups when I was using Win 98, regardless of a large harddrive and over 300 MB of RAM. WHen I got a new PC and WinXP, I never had another lockup.
Ram? The more the better with Photoshop. If you don't have at least 256MB you'll be hurting. I'm using 768MB atm, and it's handling 100+ MB files with 20+ layers fine. You might be able to ease it's load by reducing the number of steps your history remembers.
Hard Drive Size? The bigger your hard drive, the more you can devote to your scratch disk, which helps Photoshop out immensely.
Do you have partitions? If you have C and D partitions (or whatever letters you are using for them) put your scratch disk on the partition the program itself isn't on.
Page file: Make sure your windows page file is twice the size of the amount of RAM in your PC.
These should help with lock up and slowness issues with PS.
