I have a windows xp professional sp 2 with 223 MB RAM and 100 GB Hard Drive, 2 hard drives, one of 80 GB and the other of 20 GB. The problem is that whenever I start the PC after the login screen comes, I log on to my account.(I have only one user account in my pc) Instead of desktop, a black screen comes and I have to work using Ctrl+Alt+Del. If desktop comes for a moment, it flashes and the screen becomes black again.

Thank You,
Angshuman007

Try starting in safe mode with a minimal/default set of drivers. My first point of investigation would be the display adapter. However if you have recently changed the monitor set up of your computer you might find that XP lacks the driver set to allow your GFX card to interact with your monitor correctly.

How do I start the PC in safe mode? Also, I haven't changed anything

Usually to start in 'safe mode' you should hold down the F8 key at boot, this should put you into the windows boot menu where you will be able to select "safe mode"

I would also query if your monitor is functioning correctly. Consult the manual that came with your monitor to check that all is well.

Press F8 several times at boot, then select safe mode.
In safe mode, reinstall / repair your display driver.