I have a macbook that Finder won't open. I want to connect it to my windows PC so I can copy files off it so I can reinstall MacOSX.

How can I access the filesystem?

I don't know, but if you have access to another Mac and both meet the requirements, you can achieve your goal using Firewire Target Disc Mod (http://support.apple.com/kb/HT1661)e.

I know I can do that but I only have one Mac :(

The my suggestion is for you to provide as much detail here as you can, about your Mac model and version of OS , along with as much detail as you can about the issue and what you were doing/have done since (when) the issue presented itself. Then someone here may possible be able to help you save your files and/or restore Finder. ;)

I posted here: http://macosx.com/forums/mac-os-x-system-mac-software/300808-need-urgent-help.html