I am using LR 2.4 and it worked well with CS-3 now with CS-4 when I try to open a shot into PS to edit it will not do it. Also CS-4 is not letting me open any CR-2 files but it will let me open a PSD. Appreciate any help!

You should be sure that ACR (Adobe Camera Raw) is correctly installed. You can verify this when you choose "About Plug-in" in the Photoshop Menu. Look for something like "Camera Raw…". It's also possible that the installed version of Camera Raw is too old and doesn't support your current CR2 files.

I'm using LR2.4 and CS4 on a WinXP machine with 4Gb ram and 2.66Gh clockspeed with an Intel quad chip and a pair of nVidia Gforce 9800GT cards, 500mb each, run in tandem (SLI).

If I don't have CS4 open yet, when I tell LR to open a file in CS4 I get an error message after a while because it takes too long for CS4 to open. However, after it is open, the same file will open in CS4 from LR.