Paul,
You're asking for a lot of problems doing it that way. My best advice, burn all your important data to a cd. You can save your web browser favorites settings, even your cookies so web sites recognize you. In Internet Explorer it's File> Import and Export> click on Export Favorites, then run it all again to Export Cookies to the CD. Save everything you want to keep on the CD(s).
Then reformat the hard drive. Once you've reformatted it, put it in the new tower under construction. Boot form the Windows CD and do a clean install of all chipset drivers, ethernet drivers, sound drivers, video drivers, ect.
Otherwise your going to have a computer full of old goo and conflicts that won't perform at it's peak. After all, that's why your putting out the $ right, better performance.