On another note - 512mb ram is not enough for Vista. There has been two substantial freezes whilst attempting to make this post.
You might as well be trying to drive up a near-vertical incline in a Yugo towing a trailer, there...
Iain,
Those discs spin at high enough speeds that you don't want one to fail structurally inside your drive. It's not the "OMG it'll asplode and kill you" sort of thing that is rumoured on the interwebs, but a flawed disc that fails will pretty well trash the inside of the drive, and that's more expensive to fix than just buying a new game.
How old is the disc? If you bought it recently, you might be able to get the store to exchange it for a new copy.
If not, you might be able to talk the manufacturer into sending you a replacement disc if you send then the flawed one, for cheaper than buying the game again.
-BP
This is true. Materials failure is sudden, and it'll trash your PS3, if that's what you're talking about. The refusal to spin up a cracked disk is the PS3 trying to preserve itself.