Personally, if you're going to keep that thing running XP I'd disable the network, keep it off line and skip AV completely.
 
With or without AV, that computer will fall to an unpatched exploit eventually if left online.  MS has stopped security updates, leaving all machines running XP with a giant bulls eye on them.  There are plenty of folks looking to add machines to their bot net for spamming or DOS attacks.

I also use the free version of Avast on my XP machine. Works well for my internet surfing needs.
 
Mike

Warning about Avira... I was using it for ages, in hope I am protected. But recently I was browsing my archives, and then my computer was slooowed down. I have found one virus by hands, packed it into dedicated file and then tried several AVs. The result:

• Avira has found nothing in this file
• Microsoft (Win 8.1) has found nothing in this file
• Avast has found nothing in this file (it was not on my comp, so I am not sure it was up to date)

Only G-data has recognized the virus. I have scanned my computer with it and there was several more viruses in old archives...
 
Note, that the virus could not "activate" itself under Win8. I have feeling that most AV producers are concentrated on viruses which are effective on Win 7/8, completely ignoring old (XP and pre) versions.
 
G-data has one month trial, but then it is relatively expensive. Upgrade to Win 8.x is cheaper in long term 

I used to use MSE exclusively, but now I'm not so sure about it.
 
It DID let one or 2 nasties through which were later picked up by Malawarebytes, so please don't assume that MSE was ever the panacea it was made out to be.

If you want the best of the best and money isn't an issue, go Kaspersky. It has a yearly subscription fee but it's pretty much the best virus protection you can get. BitDefender is another very good one you can buy.
 
As far as free options, Avira is pretty much the best you will get.
 
Popular options to stay away from are AVG, Norton and McAfee. Even MSE is pretty useless as it has a very low detection rate. MS has admitted that MSE is very minimal and they suggest getting an actual antivirus program.