Hmmm, we have two sets of symptoms based on the description.
Less noise from the cooling fans and the system responce much slower than normal.
Spyware programs won't affect cooling fans, the fans are not software controlled. The power supply not putting out full voltage could explain the fans, but low voltage from the power supply won't make the processor chip run slowly. If the power supply is not giving full voltage to the motherboard for the processor and other items, the system is more likely to hang/freeze randomly. The processor chip does not increase/decrease it's speed based on changes in voltage from the power supply.
We still don't know what type of system it is (Intel, AMD) and how new it is. I know that newer processor chips from both Intel and AMD will deliberately drop their speed when the processor chip temperature goes over a certain level (it's controlled in the PC's BIOS for motherboards that support this feature).
If the PC has a reasonably recent processor chip, my guess is that the cooling fan for the processor chip is not running properly (less fan noise), causing the chip to heat up to the temperature limit set in the BIOS, which causes the processor chip to drop it's speed to reduce the amount of heat it produces (system now runs slower). This would seem to explain both less noise (fan not running properly) and slow system (CPU slows itself down because it's overheating).
With the PC turned off, open the case and with your fingers feel if the processor cooling fan turns easily or not. Then turn the PC on with the case open and see if the processor fan starts up properly (and all other fans as well. Careful of your fingers, it's not good to stick them into a moving fan). If the processor fan isn't running, turn the PC off and don't use it until you get a replacement fan. Overheating WILL literally kill a processor chip by cooking it to death.
It may actually be easier to get a complete replacement heatsink/fan combo for the processor chip and install it rather than just a fan, IF the processor fan is the problem. Any decent computer store should have replacement heatsink/fan combos for processor chips.
If the procesor fan is running normally, then the problem is something else. I'm not prepared to try to guess what else it might be based on the limited information you've given so far.