I have used video calling between an iPad 2 (as well as other iOS devices) and a Windows desktop client and it works on all of these...
Skype
Mirial ClearSea
VTok
Yahoo Messenger
Some of my impressions comparing these 4 against each other:
The big caveats here are the quality and cost of these clients/connections. Assuming WiFi as your best possible connection, Skype has proven to be the easiest to use with the best quality at the best price=free. As you pointed out in your earlier post though, the iOS app itself isn't all that stable so you will have to tolerate occasional crashes or erratic logouts among other problems. Mirial ClearSea works very well and the video quality is very good. However, this is a professional level server/client based application so is Not free and may be more than you really need. You can get a free demo account from within the app that will include 2 usernames to try out and is not feature limited so you can hand out 1 to a desktop participant and use the other for yourself on your iPad2 or other device. Again though, this is definitely not a cheap app when you factor in the cost of the server. VTok works fairly well and is free but the video quality is not very high compared to Skype or ClearSea. Still, it is acceptable quality and free and the GTalk desktop client is easily accessible to anyone. The other option is Yahoo Messenger; it works and is free as well but, the quality is pretty poor compared to all of the others IMO.
I ranked these in a loose order above based on quality but, remember that if ease-of-use is a higher priority or cost, or anything else they would not be listed in the same order. The main thing I would recommend is to try each of these out yourself. All are free or there is a free trial so see which you find easy to use and which has the best audio and video quality for your calls. and of course the clients I listed are specifically to connect from an iPad to a desktop Windows client - there are many more apps for mobile to mobile or iPad to Mac or other variations...