My dad's Dell Dimension 2350 windows XP home edition won't power off when you try to shut it down .

Checking your autorun applications may help. There may be a process like Norton Antivirus that is preventing the system from shutdown. Start by following the previous instructions to press F8 on startup, start safemode, then shut down as see if the issue remains. If so it's likely a hardware issue. If it does shut down normally get a program called "autoruns" it's hosted by the microsoft technet team and will list everything starting with windows. Review the results after opening it on a normal system restart and start by a process of elimination to find which program is locking up the system.

There is one solution that you can do, you can hold the power button for about 10 seconds and this will shut your computer down, this is known as a hard shut down. My question would be how often does this happen? If it happens at every shutdown I would recommend scanning your computer for viruses and malware. If that doesn't fix the problem I would recommend backing up your data and restore your operating system.