You want to keep using your DVR as a DVR, yet be able to access its hard drive from the PC? The answer is "it depends, but probably no".
If the DVR has a network port and is designed to hook up to a PC then you'll probably be able to do some limited reading of the box's hard drive from the PC. For instance, the new Panasonic DMR 500 hard disc DVD recorder lets you do this I belive, but only in a limited way. You run some software on the connected PC that lets you edit track names and playlists and so forth, but you don't actually access them directly.
If you were good at writing low-level software - probably without any help from the hardware vendor - then you might be able to write something that let you access the hard drive directly.
If your DVR doesn't have a network port then not only would you have to write the software but you'd have to design some hardware too. I doubt it would be worth the time and effort in either case.
In all likelihood the hard disc in your DVR doesn't use a file system that your PC can recognise anyway. Even if you took the hard drive out and put it in your PC you wouldn't be able to read it.
Nelviticus