Stop: C000021a (fatal system error).The session manager initialization system process terminated unexpectedly with a status of 0xc000003a (0x00000000, 0x00000000)

try pressing F8 key on startup and choosing last known good configuration.

Reload the software,.

It appears that your Windows Registry is corrupted. You can try to use a Registry repair utility like CCleane to fix it. If that doesn't work you will have to reload Windows OS (Operating System). For more info no this error code Google "c000021a".

reinstall your OS from disc this should cure this problem

You most likely have a hardware issue. Very possibly memory.
You'll need to troubleshoot that by disconnecting everything in the case but the video, CPU, and memory and see if it starts OK.
Then hook up the hard drive and see if you can boot into Windows. If you can't, unplug one of the memory sticks and try it. Then the other memory stick and try.
If you have a Windows startup (installation disk), boot from that and opt for system repair. You will want to get to the command prompt and run chkdsk.
See my tip here:
http://www.fixya.com/support/r4501608-when_windows_xp_start_primary_fix

damaged OS windows or most commom hard drive failer..

burn this disk as an .iso and boot from disk. try the recovery options on this disk first, and if nothing then you may be in need of a free OS install. you MUST keep all virus, spyware, and firewall up to date n order to keep your computer safe. free-avg.com does have a great free download.
Good Luck

Try re-seating the RAM (memory sticks) with the system turned off. Clean out any dust from inside the PC, especially on the CPU fan.

plz give your motherboard make and model and the operating version

How do you mix this problem?