Its entirely possible to do a clean install with an upgrade version of xp.
If you already have win98, 98se or me, and buy the home version, boot from the cd and it will recognise that there is already an os installed. When you get to the screen where you choose which partition to install to, choose the partition where windows was previously installed, then format it with the ntfs file system. Of course you'll have to back up everything first.
This ensures you have a 100% fresh install of windows, with no reminents of drivers left from 9x.
If you dont have 9x installed, then you'll need a to insert full version cd of 98, 98se or me when it asks you to. If you go for winxp pro, then you'll need to either have nt4 or 2000 installed previously, or provide one of those cd's instead.
Its not even neccesary to own the full version cd's of the os' required for the upgrade versions...but you didnt hear that from me.
Just borrow a copy, since you'll be able to clean install an upgrade version of xp just as long as it's already installed on the system when you boot from the cd.
The only time you'll ever need to provide a full version of a previous windows cd would be if your hard drive is completely destroyed or stolen. Even clean installing onto a brand new drive is easy, just ghost your current install to the new drive, then format it when you get to the screen where you choose which partition to install to.
Buying OEM full versions is a wank, they cost more than the upgrade versions (which dont need to be installed over a previous os, so they arent really an upgrade anyway) and dont come with customer support from ms.
If something goes wrong with it, you'll have to contact the vendor you bought it from, and unless you bought your entire pc from them they wont want to help much.
If you have anything slower than a p2 400 and less than 128mb of ram, then go for 2000, since you wont be able to run with all the extra niceties that xp comes with, but if your pc can handle it (500mhz, 128+mb ram) then get xp, its definately worth it.