Assuming you are talking about "System Restore":
On the desktop, rightclick "My Computer", click properties, click Performance, then File System. Then click the hard disk tab, then you should see a slider for "System Restore Disk Space." Adjust that to the amount you want to be reserved.
If you have some weird backup thing, like HP's next to useless "recovery" partition, then things get a bit more complicated. Reply if this is what you're stuck with...
hope this helps,
~psi42