Windows XP system restore may not work if there is an anti-virus program blocking the process or if the operating system has been infected by a virus. Insufficient disk space may also prevent a system restore from running.
System restore usually takes up between 200 and 400 megabytes of disk space on personal computers. Both viruses and anti-virus programs may prevent changes to system files and settings. Start the computer in safe mode to block viruses and other programs interfering with the system restore. Running an anti-malware program may also block malicious programs interfering with the process. If the problem persists, eliminate start-up programs.