Provided your hidden 'recovery' partition is still there and didn't get deleted during the upgrade, then the recovery will be set to Windows XP and then you can use the W7 upgrade disk.
The only problem that I can foresee is that XP is no longer supported and you would not be able to update it. This might interfere with the upgrade disk and cause it to fail.
A much better solution is to try and find out why your pc is now running slowly and correct any problems. You could start by doing a disk cleanup and disk check, followed by stopping anything unnecessary from starting during 'boot up'.