Check your Power Manager or whatever that is and see if you can set it to "max conserve". They may have the on-AC power settings for "Max Performance".

You might also try running it on AC, but with no battery installed. You could have a bad/old battery that's drawing current through the motherboard circuits. There was a battery recall, but I don't know the cause or symptoms.

Does Vista on the laptop include diagnostics to query temps and fan speeds? If not, check CNET for utilities, maybe something like this: http://www.snapfiles.com/get/siw.html

I haven't used it, can't vouch for no malware, viruses, etc. Just an idea.