Don't touch it with a bargepole. The vast majority of so-called registry cleaners don't help you (and can even damage your computer). Further, clicking on any unsolicited link can result in something really nasty being put onto your computer, which could destroys all your files (or lock them until you pay a ransom of hundreds of ponds to access them again) or allow criminals to access your online banking.
To get rid of the pop-ups, download, install and run the free version of Malwarebytes Anti-Malware:
https://www.malwarebytes.org/antimalware/
(NB: When installing, take care to de-select the pre-ticked option to take a free trial of the 'Premium' version).
If you actually do want to try a registry cleaner, there's only one which is widely recommended in the technical press:
http://www.piriform.com/ccleaner