|Is it true you can only use them once ?|
You can use them as many times as you like, but only on the computer that it came with.
If you are trying to install Windows 7 on a Dell computer that came with Windows 7 using the key on the case of computer and it doesn't work you will need to contact Microsoft.
Or check that you are using the right numbers. One character wrong and it wont work
You mention XP but you tag says windows 7. Which is it as you cannot use a windows XP code to install Windows 7 and vice versa.
message edited by StuartS