Actually, there are two solutions to this problem that I know of.
Both use RDP to connect to a background user session that can be accessed while another user is logged in at the actual machine.
One is called iRapp
"Use your Mac and PC simultaneously through a single monitor, mouse, and keyboard. Welcome to the Switzerland of computing. . . iRAPP is an application that gives you cohesive, interactive access to a Mac via your PC. You can choose to view the Mac in a blended session with Windows and OS operating systems into one window, or you may use the built in Remote Desktop Protocol to view it in a separate window. Either way, iRAPP allows you to open and work on Mac applications right on your PC within the Windows environment. Copy and paste, save documents, open programs, share files – download a free 14-day trial now!"
The other is "aqua connect terminal server"..
I'd post the URL, but I don't have enough cred.
Worldwide leader in Mac Terminal Services and Provider of the first Mac Terminal Server. Other alternatives to Mac Virtualization include Mac Remote Desktop. We Provide solutions for all industries including FileMaker and Creative Suite Users, to organizations looking to deploy Mac OS X on Windows or create multi-user iPhone or iPad SDK environments.
irapp looks to be the more consumer friendly choice, IMHO.
I tried using it and had some problems logging in, but it seems to be a lot like MS RDP, and can use the same clients.