A lagging computer can be a sign of heat.
You might first want to blow out the 4 years worth of dust (unless you already did). A can of CLEAN compressed air will do it.Check your CPU and Chassis fans for proper rotation (speed) as well as the fans in the power supply.As for your sound, you might want to go to device manager and remove all sound items and then reboot. It should find it as new hardware and that MIGHT get it back working for you.
The best way to check if it is the motherboard would be to install a new hard drive (maybe a borrowed drive?) and reinstall the OS. Upon reinstall, if things still do not work, I would say it is bad. Could be just the Southbridge Chip (that handles the sound) but it's not something you will be able to fix.
If you can get the same type of motherboard, that would be best. Otherwise, a new motherboard may fail the Windows activation and you will need to call them for activation help. Not a big deal. Just tell them your Hard Drive went south.
Here is a program SpeedFan
, that will tell you the rotation speeds of your fans and the temp of your CPU and Motherboard.http://www.almico.com/speedfan.php