I recently went through this and did a re-install of Win & for a friend. I cleaned out the dust from inside the machine, wiped the HDD and did a clean install of Win 7 pro which I downloaded and burnt the ISO image from the MSFT website. After the installation I took the Win 7 key from the machine, and validated the installation using this method. Make sure you have the product key first, otherwise your installation is only good for 30 days. My friend's product key was behind the battery, some are on a tag on the back of the machine.
Activate using the telephone
If Windows isn't able to activate online, you'll have the option to activate Windows 7 using an automated phone system.
Open Windows Activation by clicking the Start button Picture of the Start button, right-clicking Computer, clicking Properties, and then clicking Activate Windows now.
Click Show me other ways to activate.
Type your Windows 7 product key, and then click Next.
Click Use the automated phone system. Administrator permission required If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
Click the location nearest you from the drop-down list, and then click Next.
Call one of the available phone numbers listed. An automated system will guide you through the activation process.
When prompted, enter the installation ID that's listed on your screen into your phone's keypad.
Write down the confirmation ID that the phone system gives you.
Type the confirmation ID into the space provided under Step 3 in the activation dialog, click Next, and then follow the instructions.
If activation isn't successful, stay on the line to be transferred to a customer service representative who can assist you.
http://windows.microsoft.com/en-us/windows/activate-windows#1TC=windows-7
A good friend turned me onto this. As long as you use the same version of Win 7 you're golden, have a fresh install and ZERO bloatware. It was well worth the time!
Those registry cleaners can be a great way to hose an OS. Be very careful with them.