You'd have to use a floppy disc I think. Make it bootable, and then have the System32 file on it, and have it copy it over to the right directory from there.
I'm no wiz when it comes to dos commands, so i'm not sure exactly how.
Plus, if you have Windows XP (Though I don't think this file is in XP and so you can't be using it), its very hard to get a bootable floppy as far as I know. Try this site to download one, as I think they do have ones for XP if this is the case, or they have one for other windows too:
So create that, and then drag the system32 file onto the floppy disc as well from another source, from maybe the computer your using now.
After that, and the bootdisc has sucessfully loaded on bootup, you can start typing commands. This should do it:
A:\>copy system32.dll C:\Windows\System32\system32.dll
This should, if I'm right, copy the system32.dll file from the floppy to the right directory on the computer
I think its in System32 within the windows subfolder anyway.
You can check that though before doing it. I tried to for you, but when doing a search, it didn't find the file...After doing a quick search on the Internet, I realised that Windows XP doesn't include this file and so isn't needed.
Hopefully after doing this, Windows will boot.
I hope this helps
*EDIT* If you do have Windows XP and its asking for that file at bootup, i've found it could be a corrupt installation, or a virus since one hides itself as system32 apparently and looks rather innocent.
Why exactly has it suddenly deleted? Is it a new installation?