Thanks AndySD, the O/S that was installed is XP. the previous O/S was Me.
The reason I asked is that a friend has just had her computer upgraded to XP. Since then she has had nothing but trouble with it. I had heard that if an O/S was installed on top of another you may get problems.
You can get problems upgrading, recap, that's why it's always better to do a clean install. Second best is to dual boot XP with ME if there are things like older games or peripherals which wont work on XP for you, but did work on ME.