Do a couple of online scans here..

http://housecall.trendmicro.com/

and here..

http://www.pandasoftware.com/products/activescan.htm

for confirmation. The malicious sware removal tool only looks for a few specific viruses or worms so it wouldn't necessarily be the best scanner under the circumstances... sometimes even for the ones it's supposed to look for.

I haven't used NAV in a long time so I'm not sure about the checkmark being greyed out. Perhaps someone else here would have a better idea about that.