In WinXP the default is to make a restore point every day providing the PC is idle. They are also made by most programs when they are installed. You can make then manually as often as you like.
You can change the interval between system restore points by going to Start > Run, type regedit and press Enter, then navigate to registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\WindowsNT\CurrentVersion\SystemRestore. In RPGlobalInterval it is set to 86400 (seconds) which represents 24 hours. Change it accordingly.