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:\.