|Never known a virus disable fans - normally a hardware fault.|
Agree with follow up posts, dust quite likely and is relatively easy to remove. While you are in there clean case vents and CPU heat sink (probably mentioned in one of the links below - not checked).
"safe for me to open the computer back". In what way?
Before working inside, remove the main battery as well as the power - then hold the power on/off button in for a few seconds to discharge everything.
Always pop back and let us know the outcome - thanks