CPU / motherboard upgrades
Sometimes customers want to effectively achieve getting a "new computer" for cheap by simply upgrading the CPU and / or motherboard, when in reality it is anything but simple. While most upgrades (such as hard drive, memory, etc) are straight forward, CPU and / or motherboard "upgrades" can be very problematic, hit and miss, etc.
For starters most older motherboards are not capable of running newer processors. Even if they are, other parts (such as the video card) often are not compatible and must also be replaced. All Windows drivers must be reinstalled. Often the computer will not boot to the desktop without lots of troubleshooting work. Then after several hours of labor the end result is at best a machine that may do strange things and is more likely to develop problems.
Furthermore the Windows OEM licensing agreement would consider a motherboard upgrade to effectively be a "new computer". This would trigger the need for Windows to be re-authorized within 30 days, which Microsoft would not allow. So tack on another $100 for a new copy of Windows.
In conclusion it is a disservice for a repair shop to charge a customer for many hours of work to perform a CPU / motherboard upgrade, not to mention a disservice to the repair man himself who gets blamed when the computer inherently acts up.
It is more cost effective to buy a new computer.