I had accidently delete some system file under C drive:

io.sys
msdos.sys
ntdetect.com
ntdlr

what should I do?is this 4 system file very important, or i just can ignore them...?

Verify Window’s Files

1. Go “START”, “RUN” type “CMD” Press “OK” or Boot PC into “safe mode with command prompt”
2. Type sfc /scannow /f (note:- space after sfc)
3. Windows file protection will check your files. Follow any prompts; you may be required to use your XP CD.
4. Re-boot your PC

A windows repair will replace the files witch infact is needed files. Last nite I had a machine with a NTLDR missing. I didnt want to doa win_repair because I was looking for a 'workaround' just to get into windows. i was forced to do the win_repair at the end.

NOTE: when you're done with the windows repair, open explorer>tools>folder options>view. Tick the checkbox that ive circuled in red in the image thats attached. Reason being so that you dont delete the neseccery files again by accident at a later stage :)

Cheers
sittas

Mmm... these are the two that you should [must] have:
ntdetect.com
ntdlr
Copy them in from another sys. If your sys is still running don't shut it down until you have replaced them into the root of your System Drive. [usually C:\]

check the Recycle Bin and see if the os moved the files there. If they are there then you can just simply restore them.

check the Recycle Bin and see if the os moved the files there. If they are there then you can just simply restore them.

The simple is normally the best!!!!!!!!!!!!!!!