Please be a little bit more specific...
Do you mean you can't type a backslash in DOSbox itself or you can't type a backslash at all (which I would find very hard to believe)
Here's a little work around.
Open DOSBOX.CONF in notepad and look for the [autoexec] part. (it's a the bottom)
Now type
mount c C:\
Unless you're completely unable to type a backslash, this should work.
Alternatively, you could use my front end which allows you to mount with out typing anything. The link is in my profile
Cheers.
Author of D-Fend, a DOSBox frontend.
D-Fend homepage.
http://members.home.nl/mabus