Remote Access

For a number of reasons, I would like to access my IMAC (recently updated to Lion 10.7) remotely. I used to be able to do this when I was running BSD, but I haven't been able to do it with my iMac.

I can access the desktop remotely, but that requires me to be already logged into the MAC.

What I need to do is connect to the MAC from say a PC using putty, log in then start the desktop at an apropriate resolution for the pc. I can log into the MAC via putty - that works ok. It is starting X that I have a problem with - I used to use something like startx 1024x512.

Can somebody help or point me to a simple guide.

Thanks

Rob