Webuser Forums: My computer has started rebooting itself constantly

Last week, my computer started re-booting itself while I was in themiddle of using the internet. I have run my anti-virus software and it just came up with tracking cookies, which I removed.
I went on internet this morning and now it won't even let me get on the internet. It is connected but when I double click on internet icon, it reboots. I keep getting an error box up and click send error report.
The messages I have seen come up so far are:
The system has recovered from a serious error
One of the files containing the system's Registry data had to be recovered by use of a log or alternate copy. The recovery was successful.

Orange3 came up at one point on the error but not come up since.
I haven't done anything different or loaded anything onto computer.

Can anyone help me sort this out? Have I got a virus that is not being detected? Has something changed in my registry?
I have run out of ideas!

I had McAfee preloaded which came with the PC and not sure if it ever worked. I removed it when problems started last week and installed AVG Vers 8. I have AVG vers 7.5 on my 2 laptops and it works fine (one picked up a trojan and remopved it recently).

Please help!