I dont know if i've got a virus... or malware

I was online last night and i got a warning come up sayign that my computer is goign to run slower because it has been infected (those arent the exact words... it may have said what it was infected with but i really dont remember). The "popup" then continued to show me, in bright red type, that i have vruses of soem king on my hard drive and two other places. It showed there being more than one kind of virus (malware and some others)... none of which have ever been picked up on a norton scan (i have norton internet 2008). The "popup" then asked if I wanted it to fix the problem for me. By this time i was thinking that this may be a hoax and by accepting i would be actually downloading a virus (i think this is what actually happened to my last laptop, and by the time the virus was done the laptop was pretty much beyone repair). So i clicked tried to "x out" of the program or whatever it was that opened. Then windows asked if i wanted to allow the program to run (and it was an antivirus program, like i thought it may be). I selected "decline". After this i ran scans with norton but it only scanned about 7000 things, and im sure there are more on there. The thing is though i dont know if it was scanning less items before the virus inccident occured last night. Either way, after scanning those files it came back with only one trackign cookie found. I then found Windows Defender (whihc i didnt even know my computer had.. it was turned off and ive never used it.. so i turned it on) and ran a quick scan and then a full scan. Windows defender found nothing. Im sure both norton and defender are upto date, or as upto date as either program could be as of last night. That said, im still really nervous that something is on my computer that shouldnt be. I then shut down my computer and havent turned it on since.

 

I am worried about downloading any antivirus searching materials seeing as though i seem to have bad luck on choosing wich ones will give me viruses and which wil acually help detect and get rid of them. And i really have no idea what im doing when it comes to viruses. How can i actually tell if theres soemthign on my compouter?

 

Can i please have osme advice on what to do?

 

Thank you,

 

                  Samm