When i start my pc,

Press F8 when PC starts, to choose another Operating System.

There you will have the option to choose:

"Last known good configuration"

Try that.

If doesn't work, using the windows XP CD, put it read CD first on bios, and just go forward until you see the message to install it again.
Be careful to not go to the Format part...

Windows will proceed to re-install XP again, with default rules.

If that work, you may after that, reinstall Anti virus software (AVG or Kaspersky it's the what I will recommend) and run it.

After that sometimes you will need to re-install some software, previous installed but not working anymore.

Also you can see here if this will help you:

Solve 1

Good luck

For that when you poweup the computer press F8 key then enter in safe mode.
Then go to start menu then run type in run "chkdsk /r"
then press "y" then restart computer then it fix error on harddisk.
But if you not able to enter in safe mode then you need recovery disk then boot computer with that disk.
For that when you power up the computer press F1 or F2 or "del" key then in bios screen go to boot menu set first boot device as cd/dvd rom then save and exit.
Then put disk in rom then it boot with it.
Then follow the procedure and repair windows.
Let me know if you need further assistance.
Thanks.

1,remove all software you installed last
2,install microsoft security essential software from the microsoft website
3,restart the pc

switch on your pc and then keep pressing F8, until a menu appears choose safe mode and see if the computer boots up. If it does boot up you can then remove whatever it was ,you installed to cause the problem if you dont know what it is, then click start > all programs > accessories > system tools > system restor and select a time when you know for certain that your computer was running correctly. If this doesnt solve the problem then you will need to repair it using an original windows disc