Tools stop working in Photoshop?

Hello, I'm sorry to bother everyone, I know this is an icon tutorial community, but I'm hoping that with all the knowledge here about the use of Photoshop that someone might be able to help me. I started using Photoshop CS a few weeks ago and noticed that a few tools quit working after using them. Most notably, brushes. There is no set amount of time that it takes for this to happen - I could just open Adobe, apply my first brush, get ready to apply my next one, and it stops working. All I get is the arrow. Or, I could use five-ten brushes and it quits working. Sometimes I can't apply brushes at all. After a tool quits working, I'm unable to use any other tools such as gradients, text, etc. Scratch disks are not the cause since I have them set where they're suppose to be. I've tried installing different versions of CS, and even just using the default brushes, thinking that the brushes I'm trying to use might be corrupted, but that's not the case since this happens with the default ones, too. I thought I could get rid of the problem by switching to CS2, but the problem is still there. The only version that doesn't do this (so far, knock on wood haha), is version seven. Does anyone know why this is happening, and how I can fix it? I'd appreciate any help anyone can give. I'm kind of desperate here. ;\