Popups that tell you that your phone has a virus are usually browser popup ads that are trying to lure you into installing some bogus antivirus app that will probably be malware in itself. I would suggest clearing all of your browser's cache, history, and cookies--in Chrome, you tap Menu>Settings>Privacy, then tap Clear Browsing Data at the bottom. Then be cautious about the websites that you visit.
In Chrome, also make sure that you have "Block Pop-Ups" checked on in Menu>Settings>Content Settings.
What apps keep crashing? These could be due to a virus, but true viruses are pretty uncommon on Android phones--you have to agree to install the piece of malware in order for it to do damage, so unless you've been installing apps from shady sources (like filesharing sites) or poorly regulated appstores (pretty much anything besides Google Play, Amazon Appstore, and maybe a couple of others), you should be ok.
You could always try a system cache partition wipe to see if it helps the random crashes. It won't erase any personal data.
http://forums.androidcentral.com/sa...ing-into-recovery-mode-galaxy-s2-s3-tab2.html
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