Local disk C: is filling up...

I've got my Windows installed on C:
It's 50GB partition, and the used space is increasing day by day. Only my programs are installed there. I'm using some file cleaners to clean the temporary files, but still. Just a few days ago i had 13.7 GB free space and today i have 13. Is that normal? If you can tell me, what are these files used for? Do I need them and if not - how to get rid of them? :)
The programs I'm scanning with are the Windows Disk cleanup, CCleaner and Your Uninstaller (which is actually pretty good in that i think).
Probably there are some threads about that but I couldn't find them, so I'm sorry if it has already been discussed. You might just give me a link to the other topic.

Thanks.
Pavel

The restore points are limited to 5% (2.44 GB) of the drive. I think it might be because of them, because after i deleted every restore point i freed up 1.6 GB, so with every installation/deinst. it will take more space, and I've done a few the last couple of days.
I don't know what page file is so i can't tell you about it. I've heard it but don't know :)
As for the System restore, I'll decrease the disk space usage to 1.46 GB.

Do you think there might be other reasons for the free space to decrease. I have to say that it's not that quick, but it is. After i was done with the installation of all the programs i had something like 17 GB.
Btw i've just installed .net framework 4. Do you know how much space did it need?

I agree that it sounds like the disk space you see filling up is due to system restore. Here is a resource that explains how system restore saves: How much disk space does System Restore require?. Have you tried defrag: Improve performance by defragmenting your hard disk

Cheers,
Cassandra
Microsoft Windows Outreach

OK, how can I explain the less free space then? What can I do about it? Can I be sure that after 3 or 4 months I wont have 5 GB free space? :)

I have disabled all the services that i don't use (and I'm certain of what they do), using a guide in internet.
Now i remembered about the page file. It's the virtual memory. It says that it's recommended to use 6 GB, but it's set to 4. I'm not sure if I've changed it or it's by default like that :) I have 4 GB of RAM, yes.

I don't mind that the free space is only 13 or 30 or 5 GB free space, as long as it works fine. I don't use that partition for anything else than the OS and the programs installed, and i have all the once i need there. I've got one HDD, devided in 2 partitions: the one with 50 GB for the windows and the programs, and one with the rest (about 180 GB) for all my stuff. My concern is if it needs this space, maybe it will run out of it sooner or later at it will need more.

The only big programs i have (and i use them often) are AutoCAD 2010 and Revit 2010. All the others are small programs, probably less than 50-100 MB each. I don't have any games. Ah, and the MS Office 2007 (word, exel, ppoint, project and visio). I'm not aware if the drawing programs make big temp files, but I don't think so. I will ask the IT manager in the university though.

So can you give me your opinion on how to set up properly my CCleaner - what to check and what not, and if there is something in the tools also?

Also what's your advice, guys? Should i change the location of the restore points and the page file to D:? Will it affect the computer's performance? As i said - I don't mind that they use the C: , as long as it doesn't run out of space.

PS: Is the defragmenting going to free up some space? I do it about once per month.