You can keep a retail copy of Windows 7 (upgrade or full version) to migrate to any computer of your choice as many times as you want, as long as it's only on one at a time.
If you want to return the laptop to its factory condition, run the Recovery Partition by restarting and tapping the key given on first boot screen or in the manual on the Support Downloads webpage for your computer model. If this partition was wiped or can no longer run, use the Recovery Disks you should have received or made when you got the laptop.
If you don't have Recovery disks, you can order them or use any Vista Retail installer for your version to reinstall with the Product key on COA sticker.
If you decide to sell the laptop with Windows 7, then uninstall the programs you dont' want in Control Panel>Programs and Features, or using Revo Uninstaller in Advanced mode to get it cleaner. Then delete the contents of your active User files (Documents, Pictures, etc). Unhide Hidden Files in Control Panel>Folder options>View tab to delete the contents of the Appdata folders which hold your program settings.
Then clean up the HD with CCleaner and defrag with Auslogics Disk Defragger.