G'Day
When was the last time you deleted your temp internet files and temp folders? How about the recycle bin?
To empty them just open My Computer and right click on your hard drive, then left click Properties, then click on Disk Cleanup (it will take a while and will seem to be doing nothing), put ticks in each box and then click on OK. Before cleaning up you can also go to the Other Options tab where you can uninstall windows components you dont use, uninstall programs you dont use and delete all but the last restore point (XP), then OK your way out. Windows will tell you have much disk space you will recover. Do the same for each disk you have, if necessary. You will not lose anything windows cannot replace if necessary.
If you have never cleaned out these locations, go and make a cup of tea/coffee - disk cleanup will take a (long) time.
If you use the above you will not have to turn off System Restore, however to find System Restore right click on My Computer then left click on Properties then the System Restore tab. You can decrease/increase the amount of disk space, on each disk, by clicking on the drive then clicking the Settings button and moving the slider to the new setting you want. There is a lower limit of 200MB (I think) at which point Restore will be turned off on that drive. You can also turn off Restore for that drive as well here. OK your way out.
Geoff