We ran into an interesting issue with the Merge tool for TFS Source Control and McAfee. None of the buttons were showing when the Merge tool was displayed. To say the least, this made merging difficult. What we discovered is that in version 8.0i McAfee's Buffer Overrun Protection messes with the Merge tool (I believe 2003 was also affected but I am not sure which features). If you come across this, disable McAfee or uninstall it and see if the issue goes away. If it does you can patch your local copy of McAfee. There was a patch out on the net, but it has gone the way of the Dodo. You should be able to contact McAfee for more information. Luckily, we have it on our network.
Happy Coding!