Serious Problems Service Pack 2

I've got serious problems with my new Dell Dimension 8400. I work away all week and it seems that one of the family has applied the Service Pack 2 download and possibly an update to Macafee Security Centre. I arrived home and the machine boot up with an C++ runtime SVCHOST error saying the runtime has requested it to terminate in an unusual way. This happens about three times during boot up and is very slow. On logging in I get a Mcsheild error saying the system resources are low and it could not start. After about three minutes I get an RPC error saying the machine will close down in 60 seconds. If I run Shutdown -a and check my services the status of RPC is 'Starting'. I cant start the service. In fact, I can't start any service, such as the new Security Centre installed by Service Pack 2. I have tried to run System Restore but nothing happens. I've tried to run the repair utility by booting from the CD but it says it can't find a hard disk. So I'm basically stuffed.

Any help would be greatly appreciated.