Did you register your Photoshop CS?
I know that if you "borrow" a copy of some Adobe products and install them on your computer, it links up to the net and tries to make sure that the copy is registered to you. If not, it quits.
I had that happen when I installed Adobe InDesign 2.0 -- it worked for a while, but then came up with the same error. (But when I unplugged myself from the net, the program worked fine, so I knew it had something to do with my "borrowed" copy... so I needed to buy it).
Eventually, I purchased the entire CS suite, and it works fine. I also registered it.
If you own a copy of the software, you may want to try re-installing it, and registering the program with Adobe.
You can also call up Adobe and see if they'll help, or search their web site for help.
If you are "borrowing" the program, you may be out of luck. Adobe seems to be cracking down on "borrowed" copies of it's program, especially anything that's new. Sometimes unplugging the net will solve the error problem.
If you are worried about costs, you have several options:
1. Buy Adobe Photoshop Elements.
2. Find a way to get a student discount.
3. Or stick it out and buy the program (it's worth it).
I use almost all of the programs in the Creative Suite, and I think it's an awesome program. I like InDesign a lot more than Quark XPress, too. Illustrator is fine for making illustrations, and Photoshop for fixing up photos. GoLive is good, but I still end up hand-coding within GoLive anyways, so I just turn on TextEdit because it's a faster load (I keep the page up in firefox for the preview; I just press reload when I want to see what things look like). Acrobat professional loads much faster than acrobat reader, and it has more functions, like security.
Good luck,
--nate