Just a bit more info to back up what ACtheTroll says.
Computers have RAM or main memory. For Windows XP then 512Mb is probably the minimum for it to work well.
Vista requires more and most people say 1Gb is required for Windows.
Now the problem is that most laptops come with "basic" graphics and have no extra graphics card. Normally a graphics card has it own special graphics memory.
But if you dont have an extra graphics card and just basic graphics there is no graphics memory, so it has to use some of the RAM.
A basic graphic card uses about 128Mb or 256Mb of memory which it "borrows" from the RAM.
So that means your 512Mb of memory is now down as low as about 256Mb.
And 256 Mb of memory was not enough to run Windows XP let alone Windows Vista. Try to do more than two things at once (play music and look and the internet) and the laptop will grind to a halt.
So I would advice AGAINST getting any PC with 512Mb of memory, either get the shop to put in more, or be prepared to buy some extra memory yourself.