If you don't have a CD capability for installing MCE on the new hard drive...I would not do so yet.
The upgrade version of Win 7...executes a clean install of XP...but it does a search of the system before to see if the user actually qualifies to install said upgrade version of Win 7. What I did was leave my XP on one hard drive and installed Win 7 on a second drive, no problem. That seems like it would be a good procedure for you to follow. After Win 7 is installed on the new drive, you can just delete your XP install on the old drive and that takes care of any viruses and your now-useless MCE install.Skip all that , that applies only to desktops and you have laptops
Although some of us have Win 7 installs on our systems...you can probably get more/better info re Win 7 situations by posting in that forum.http://www.bleepingcomputer.com/forums/f/167/windows-7/