If you've recently updated Adobe Reader and/or Flash Player you may have omitted to spot the (annoyingly) pre-ticked option to install a free McAfee scanner product, which then gets installed as part of the update routine.
If this is the case then the AVG and McAfee software will be conflicting with each other. The computer may begin to behave in a way which you associate with a virus infection (I leave that to you to judge).
Use Windows Add/Remove programs to see if there are any programs listed which you have no recollection of installing.
Make sure your computer is completely disconnected from the internet when the uninstall step leaves it with zero virus protection. A couple of minutes exposure is all it takes.
If you haven't visited the Windows Security Essentials page yet, you will have not seen the part about Windows XP support being withdrawn on 8th April 2014.