How can I re install my D drive?

I have an ADVENT desktop PC.
After a nasty virus last year I have been left with no D drive.
When you switch the computer on, you get either of the following messages:
Your NTLDR is missing
or
Could not find drive
If I keep pressing ctrl alt del it does eventually come on, but it has now also started to crash sporadically.
Does anybody know what I need to do with it?