Computer that won't seem to startup

Hello,

My brother has a pentium3 with "windows 2000 pro" for an OS with 128MB RAM and he was having problems with viruses and was experiencing an EXTREMELY 'slow computer', eg.(slow application startups, slow to get an internet browser, system freezes, etc).

He bought a Norton's Anti-virus software package the other day, and ran the scan, and it supposedly fixed his computer. His computer seemed to be running noticeably quicker! Shortly after this, he left a chess program running and came back later to find some message on the computer saying something about not being able to find a boot file or something related to a 'boot'. Later that same day, he started having problems again: the computer would all of a sudden restart by itself, and try to reboot itself for some reason, and a blue screen appeared with a message about not finding an operating system.

Eventually, after repeating a reboot cycle, it would get to the windows startup screen and he could access the internet.

Today, the computer can't even startup, instead a black screen saying:

Disk I/O error: Status = 0000103F
NTDETECT failed

a disk read error occurred Press CTRL + ALT +Del to restart

then pressing the restart repeats this loop, sometimes getting another blue screen which says:

1962 No operating system found.
Press F1 to repeat boot sequence.

I have a few questions:
Is it possible to get the computer back to 'normal' without needing to create a boot disk? My brother's computer never came with one because he bought it used. What do these messages mean exactly? Was the harddisk corrupted somehow? Could Norton's Antivirus have caused this to happen or do you think it could be the Operating system?

We would appreciate any feedback regarding this problem.

Thanks from
Victoria, BC