A System Restore is a tool used to take your computer back to an earlier point in time. For example, If you have recently installed some software which is rendering your system to be unstable, you can restore the system to a "checkpoint" which will take your computers state back to that point in time. Checkpoints can be created by yourself whenever you deem your system to be performing well, and they are also created automatically during key changes to your system i.e after a program has installed or after a Windows Update.
To perform a System Restore follow these instructions -
Click Start > All Programs > Accessories > System Tools > System Restore
Follow the instructions, but make sure you select a restore checkpoint which dates before the time you started experiencing the problems. If you are not sure of this then select the earliest date, as a System Restore can always be cancelled afterwards.
Please note you will not lose any documents/files etc but you may need to reinstall any programs or software drivers you have loaded yourself.