Do you mean remotely controlling, or just remotely accessing files?
If the first, then in theory all you need is a VNC server running on the PC (I use TightVNC) and VNC client on the handset. The only one I tried (can't recall the name) refused to connect for some reason, but I wasn't bothered as I can't imagine needing to control PC from phone when I have a laptop that does VNC in.
If you're just after accessing files, then I have found one of the easiest is ftp. I run Cerberus FTP server on my desktop and AndFTP on the phone. This allows me to set user names and logins for my kids, me, other people, with different "root" directories for each and different up and download settings.
I have access to every drive on the PC. Can't directly open files - you have to download them to the phone first and then open them. Great when you forget a document or want a specific mp3 or something.
All the above software is free.