iMac: first won't turn on now fans won't stop

Hello,

My mother bought an iMac a year and a half ago. Today her imac wouldn't turn on, so I guided her over the phone on how to reset the SMC and the PRAM.

Now the iMac turns on and works apparently fine, but the fans are wild crazy.. working at max revs.

Since I'm away I can't check much of the hard itself so I'm working blind here, just what I can do thru remote control.

I installed Fan Control and see that apparently the heat sensors and fan sensors aren't working, because they show 0ºC and the fans at -1 rpm.

Any ideas I can try software-based?

I'd be quite frustrated to have my mom's imac die after just 18 months, specially since it took me YEARS to convince her to switch to macs. And it's not like she uses it a lot, maybe a few hours every day, and just for light internet browsing or using ms office.

thanks