If you mean upgrading XP, you should find you will be ok - but backup just in case. Vista will install an option to roll back to XP, but better safe than sorry.
But you can do a full install of Vista, which will wipe the driver clean.
Unless you have a VERY good reason to upgrade to Vista then DONT.
If you had a car with a reliable engine would you rip it out and replace it with a brand new untried engine ?
There are many problems with upgrading to Vista.
When i did it on one my spare hard disks I lost all sound and could find no drivers for Vista so had a working Vista with no sound.
Some hardware has no drivers, some software does not run under Vista, your machine may not be powerful enough to even run it (it requires more power than XP).
Buy a new hard disk and put it on that but DONT upgrade.
As VHG says, unless there is some compelling reason (unlikely at the present) to upgrade, then don't! I have a test machine on which I am running Vista, and after running it for 2 weeks, I'm still experiencing problems - up to 10 a day!
What's more, almost every update from MS (and they are frequent - makes you wonder if they should have finished writing it BEFORE they put on sale), causes more problems.
Even Windows ME was more stable than Vista, and that's not a dig at buenchico :-)