NAV may have defaulted your security settings. I assume that you are using the ever popular OE, or I'm sure you would have moted otherwise.
Click Tools>Options>Security and make sure that the "Do not allow attachments that may contain a virus" is unchecked.
As for removing NAV, it is pretty clingy, check with NAZ FAQ's to find manual removal instructions, I've seen them but don't have time to get you a link right now.
You may want to follow (after stripping NAV) by installing AVG Antivirus free edition. although the software is free, it is much mroe robust yet uses fewer system resources than NAV and doesn't try to take over your PC's. http://www.grisoft.com
Good luck, if you still have problems, a little more detailed info would help.
H.S.