Slow Macbook Pro Early 2008 Running on Mountain Lion (Possibly due to GPU Failing)
Yah this is one of those errors that likely mean a hardware failure and generally speaking the only way you are repairing a failure like this (unless you are into completely disassembling your Mac) is via an Apple Store or an AASP (Apple Authorized Service Provider).

Failing all the suggestions above you may need to (first: make a backup) make a Genius Bar appointment and let a tech that does this for a living run some diagnostics. I believe such diagnostics are free, if they can do them in-store.

If they can't do them in store they'll tell you and give you the cost. An AASP might be able to beat the price. you could call them with the Apple Genius Bar diagnostics for a ballpark estimate.

Unfortunately there is no great solution to this one. Unless you can afford something new...