System Restore lets you rewind configuration changes and other important system changes to your computer. (It does not let you rewind other changes.) It keep 'restore points' for the last couple of big changes, and makes new ones every 24 hours or so, when the system is idle. All well and good, but it sucks disk space in storing those.

First, right click on 'My Computer' and choose 'Properties'. 'System Restore' is a tab. There are some very safe things we can do here, and some risky things, and one or two we don't recommend.

Well, a very safe thing to do is to lower the Disk Space. For some reason, the amount is specificed as a percentage of size of the drive, so as hard drives have been getting larger, the default percentage has gotten larger and larger, despite larger drives not having more system files. (Windows XP is basically the same size it was five years ago.) On a 80 gig drive, the smallest you can set it to is '1%', which is almost a gig, and it gets even more absurd at 300 gigs. One gig is larger than all of Windows, much less the parts System Restore protects, so it's rather unlikely System Restore will ever need that much space.

So reduce it to as small as you can without going under 500 megs. That's more than enough room to store quite a lot of OS changes and updates.

And then you can also completely disable system restore on certain drive. This is a good option if you have, for example, a 'data drive' or a 'data partition'. (Partitioning is a good idea in these days of 300 gig hard drives, but that's another discussion for another time.)

There aren't any 'system' files on there anyway, so Windows won't be able to roll back changes, but it will waste space with restore points nevertheless. They'll be very very small, but it will make up for it by keeping a huge amount of them. System Restore is only useful on system drives and partitions, feel free to turn it off on anything else. (If you put the swap file on another drive or partition, it doesn't count as a 'system file' for these purposes. It is a system file, but you don't need to, and can't, roll it back.)

And now the semi-dangerous option: If your system is working, you can delete all previous restore points. To erase all restore points, turn off system restore, and say yes to the prompt. Then turn it back on. It will make a new single restore point.

Note that some Microsoft software, like Internet Explorer upgrades, cannot be uninstalled and must be removed by rolling back to a previous restore point. Some non-MS software has to be removed this way also, especially low-level utilities. You can tell because, during the install of such software, it will pause and explicitly create a restore point. Do not erase your previous restore points if you want to uninstall those things.

OTOH, if you installed them months ago, you probably don't want to restore anyway, as you've probably installed other things since then and don't want to rewind months of computer usage. (It won't delete your documents and saved files, but, still.)

So, basically, if you're in an emergency and need some space, flip System Restore off and back on. It's not that dangerous, but something could happen in that short amount of time, or there could be something already wrong that you haven't noticed yet, like a slightly corrupted registry which is working for now but, nevertheless, will not boot, so without previous restore points you're screwed. But it's mostly safe.

And, most dangerous of all, you can disable System Restore entirely. Don't do that.

If you, for some reason, choose to disregard this, don't do anything. Don't install programs, don't update Windows, don't do anything until you've offloaded some data and can turn system restore back on.