"you've Won A ............." Pop Ups

[peachybabe]

There are several sites where a pop up box appears in the middle of the screen, telling me that I have won something, and to click ok to claim it. The only way i can get rid of it is to reboot. My feelings are that these sites are dodgy. Am I correct? And if so, what can I do to stop them?

[Old_Geezer]

It may be dodgy ads from whoever supplies those sites, in which case don't visit there. But also check your PC by scanning with Malwarebytes.

[murraymints]

and install an unmentionable blocker ...I didn't say that !

[Buenchico]

As O_G suggests, you need to download, install and run the free versionof Malwarebytes to get rid of the malware which appears to have been dumped onto your computer. (I'm assuming that you're using a Windows PC here, rather than, say, an Android tablet - it helps to know these things!):
https://www.malwarebytes.org/antimalware/
(NB: At the end of the installation process you'll encounter a pre-ticked box against an option to take a free trial of the Premium version. Make sure that you remove that tick before proceeding further!).

[peachybabe]

Many thanks all. Advice taken and used.