The OneCare backup backs up your data only.
If your PC won't boot, use the manufacturer's process to recover the operating system and programs, reinstall OneCare and restore your backup.
If you believe that the problem is simply a deleted system file, you may be able to perform a repair install of XP. See this link for details: http://www.michaelstevenstech.com/XPrepairinstall.htm
-steve