Anti-Virus with the lowest consumption of system resources.

I have a 10-year old laptop, an Acer Aspire 4720Z. It's designed to run Windows XP effectively, though runs Windows 7. It's not that great to use for heavy work, since at the maximum frequency of 1.8 GHz, it's easy for the CPU to go up to 80 Celsius. It hasn't shut down or had any malfunctions because of that, but I have to keep making sure it doesn't over-heat.

With that out of the way, I'd appreciate if someone could recommend me an anti-virus with the lowest impact on system resources. I have used MSE, and while it didn't hamper much, it would easily consume a load of RAM and CPU at times. Now, I tried Avira, but the same thing happens. I understand that real-time scanners are heavy on the computer, but I can't let it hog the CPU all the time since the CPU gets hot and, as always, the PC gets unresponsive.

Therefore, I'm looking for an anti-virus which can run on this laptop without causing the CPU to go haywire. Help appreciated.

Thank you for the reply. It's a laptop, so adding fans is not possible. Opening the case is, however it'd require going to a computer repair shop, which is nowhere near my area. I will try Panda, though. Thank you.

MSE usually doesn't hog the CPU, but at times, say when starting the PC or during opening files (sometimes, not always), it'd hog the CPU. Otherwise, MSE was running fine. Avira tends to hog the CPU all the time, which makes me wonder if they're making anti-viruses or CPU stress-test programs.

Thank you for the replies. Will check Panda out.