virus changing all my files so I can't open them

Hi, I originally posted on another thread (my original message is included below) and they told me to repost here. I haven't done the steps yet because I had a couple questions first. Will it be possible to restore my files at all or are they just gone forever? Since posting earlier the virus has now ruined my excel files also. All my files end in the word 'kanlgum'. I was also trying to back stuff up and now all the files on my external hard drive have the same problem. So I am wondering if I should just reload my computer if all my files are forever gone? But if I can get them back, that would be really good because I lost some really important stuff that I now don't have backed up anymore. Also, what is a good antivirus program to use? We have several computers and I need a good program on them. Below is what I originally typed on my first message:

Hello, I have a computer that still has windows xp on it and it has been having some issues lately. I think it may have picked up a virus. All my jpg pictures have been changed and now they won't open. They now all end in jpg.kanlgum. Do you know what this is and is there a way to fix it? I was using avast and I deleted that and now I am trying the 30 day free trial of AVG and it keeps popping up constantly saying it found something. I ran a full scan and it took 2 days to get to 96% done and then had an error. Any advice on what I should do? And what is the best antivirus program to use?

Thanks, Deanna