Tony

Is your PC locking up, or rebooting itself, or actually powering itself off ?

Operating System (Windows) problems usually result in lock-ups (PC is still powered on, but will not respond or do anything), or self reboots (usually with the infamous Blue Screen Of Death - BSOD), but complete power downs usually indicate hardware problems - power supply overloads, bad power switch or power connection, etc. Bad power strips and surge protectors are notorious for this.

Just to be clear...Fix-My-Pc and others like it advertised on television are NOT respected programs. Stay clear of them.
Eddie

Tony have you tried a System Restore to an earlier date when things were working. It isn't a big deal, only takes 10 mins and doesn't require reinstalling anything. If it doesn't help it can be undone again.

Tony,

Windows has a built-in utility called System File Checker. If you have found that using Safe Mode keeps the PC alive longer than Normal Mode then use it. Then type sfc in the Start Search Window and then double click on the sfc.exe file. Let it run. It checks every Windows file including Microsoft drivers and replaces any that are corrupt or missing.

Windows has a built-in System Restore facility. Start/Control panel/System/Advanced System Settings/System Protection/System Restore. Then you should have a choice of dates to restore from. Select the most recent and follow the instructions. If that does not improve the situation you can go back further and try again.

Ian