Hello Stiggy:
Thanks for your response... my OS is XP Pro and when I tried to activate FSX after putting it into minimize mode so that I could activate Plan-G, after Plan-G came up I then tried to bring FSX back up from the task bar and nothing would happen.... after several minutes of this negative response I did a "Control / Alt / Delete" and the message there was FSX not responding.... so, I went back and uninstalled .Net 4.0 and all appears to be good so far using .Net Framework v3.5 SP1 as called for on page 3 of 68 of the Plan-G User Manual.... Bear in mind Stiggy, that I really don't know how to use Plan-G as I installed it yesterday for the first time and it looks like a real good package to work with..... Tim, if you are reading this, my answer to you re my OS is above.... sorry for missing it in my last response to you. 
Regards
Wolf624

Tim:
Further to your reply re: .Net 4.0..... I tried to start up FSX and Plan-G and the system crashed to desktop.... I then followed the Microsoft instructions for uninstalling .Net 4.0 through Control Panel and uninstall went well... in a nutshell, .Net 4.0 does not appear to work.  Looking forward to flying using Plan-G.
Regards
Wolf624

Hello Tim:
thanks for your rapid response.... have not had a chance to try Plan-G with FSX as of this time as I am still completing the install but so far things appear OK... but normally at this point "Enter Murphy's Law" ... what I have seen of Plan-G so far is very impressive and I am hoping that it works as well as it looks..... will keep you up-dated on the Plan-G vs .Net 4.0.

Regards
Wolf624

The Plan-G User Manual calls for the use of .Net Framework v3.5 SP1.... will .Net 4.0 work with Plan-G as well?...... or is V3.5 SP1 a mandatory program?  Hope to hear from you folks soon. I have .Net 4.0 installed and that is why I aasking.
Regards
Wolf624