
I've been trying to tie down the seemingly random nature of System Restore. Sometimes restore points get created and sometimes they don't.
This article throws a bit of light on the matter. I'll have to read some others.
Rather that relying on a Restore Point being made automatically, it might be best to create one manually every now and again. Certainly do so before installing anything. Then, if you need to get back to a time before any changes took place it will be possible to do so. It will also act as a safeguard against browser hijacking (particularly relevant to IE users). Just roll back to a time before that unwanted toolbar of duff homepage installed itself.
If the computer has been running well with no glitches for a good while, there is an option to delete all but the last Restore Point to free up disk space (if you want to do that).
Viruses can reside in the System Restore directory and most virus software instructions recommend that System Restore is turned off for a full virus scan. Unfortunately, this means losing all restore points.