Free system disk space going out by itself

Hey! I'm back, sorry for delay.-
I've been quite interested in the resource monitor, kind of like it. How many things go on without knowing, he he!
Yes, I went from less than 200MB to 3,5 GB. Just the windows explorer info. Today I'm about 2,27 GB right now, and the use of the box has been quite the same. Just listening to music, thunderbird(which stores just one mail account in C drive), and some minor surfing(for this investigation purposes I try not to use the machine so much).-
As for the system restore thing, I have it enables just in C.-
Say, it's really weird that the space comes back by itself(Where was it? Where did it go?), that's what leads me to want to know what it's all about. Like, where windows stores temp data, or flash data, when I see pictures or use a program like photoshop. That's more or less what I have in mind when I ask for opinions. I know you guys are the ones that know about this stuff.-
Thanks again for your time and answers, I appreciate them. Will check those apps to monitor, they sound good. I already use some of the sysinternals progs for monitoring(you know, like disk monitor or process explorer), but the ones you post your pictures here are interesting indeed.-

Greetings!

Use WinDirStat or similar product. Run it when you space is gone and see what is consuming the space. It should be pretty obvious, but if not run it again when you the free space returns and compare. Otherwise, it's all guessing here.

Thanks for all your answers. So far I have 730 MB left, and the same environment. Except I have used a few times photoshop or played some games. But this was during the las two days, and I had 900 mb by then. Still don't know where the space goes, or how to prevent it from vanishing.-
Great info on photoshop Callender, thanks. It's also a good idea to compare situations with one of those space-progs, grateful too. You can close the thread if you like for I don't think I'll be coming back, situation is quite the same =/ Oh, and I am not using defraggler or one of them. And just use winamp for music purposes.-

Greetings!