How can this computer be resuscitated?

A friend of mine is having a few problems with their computer. Basically it is Windows XP and they were having a few problems with things crashing and hanging and their computer completely shutting down. They also get the chkdsk alert popping up quite a lot.

They have carried out a defrag, used CC cleaner, run the chkdsk several times and this morning carried out a system restore as they were unable to do pretty much. Since then they cannot connect to Yahoo Messenger, Skype has lost its sound, and even playing things on Youtube has ground to a halt with Flash not working. Finally, McAfee has just installed itself for no reason.

So I guess what I am asking is…what do they do to fix this? (system restore wont work as it stops and says something about cant read the memory)