Recover XP OS

Recover XP OS

A few days ago my pc was working fine, but now when I go to boot up it tells me that "\windows\system32\config\system" is missing or corrupt. The black screen tells you to reboot from the original CD select "r" for repair. Selecting "r" takes you to a Dos prompt and I don't know what to do at this point.
I do not have the original cd.
Please help!!!!

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