I need to replace a pre-installed Vista with XP. Is this something I could do myslef or do I need to call in an engineer?

My guess is that Microsoft will not let you install XP over Vista so you would have to reformat the harddrive first.  If you don't know how to do that, it would be best left to someone who does.

Thanks!

Some computer makers, like Dell, offer a "downgrade" service, that somehow will also preserve your ability to "upgrade" back to Vista in the future.  If the PC is a major brand, contact their support website and maybe they can do something for you.

Alternatively, I have heard of large PC stores offering a similar service, independent of the brand of PC.

However, even these services might format first, so be sure to have backups of all personal files, CDs or installer files (usually *.EXE of *.MSI) for programs.

Note that this is a good reason for keeping personal files on a separate partition, say D:\, while the operating system and installed programs are on C:\.