Yes, it's fine to use a blower.
Yes, it's fine to use a blower.
It's better to dismantle and clean it manually with a blower. I don't know how important this is right now, but when using a blower make sure the fan isn't spinning. Yes, you can, sort of... But are you certain it was just dust? Maybe try a test with Cinebench and watch the temperatures to check if it's being throttled thermally.
It seems dust buildup is the main concern. Using a compressed air canister should work well for cleaning a PC. Other types of blowers likely won’t help. If the machine is very old—about ten years or more—the CPU thermal paste might be degraded and need replacement. Also, make sure your fans are running properly when the computer starts.
Make sure to include the northbridge and southbridge heatsinks (also known as chipset heatsinks). Apply thermal paste on them too. Working with old LGA 775 boards means starting by cleaning everything and replacing the NB and SB thermal pastes, since the paste from ten years ago won’t work anymore.
Using a fan is the safest and most effective method for cleaning a PC. When using compressed air, be cautious not to blow too forcefully, as it could damage components. Keep your fans stationary while blowing; excessive speed can create enough power to harm parts. This advice applies particularly when working with compressed air.