Did you find a solution? I was having the same issue. I am on vacation connected to the wireless at the resort. If I disconnected, Word and Excel files open fine (both 2003 and 2007). Otherwise, they would take 10-15 minutes -- no matter how I access them (from Windows Explorer, form the program, etc.). Powerpoint was unaffected. I "disconnected" from my mapped network drives, deleted the "recently opened files" lists, etc., but the only thing I could do to open the files quickly was to disconnect from the wireless network here.

The solution was to set Adobe PDF as default printer -- because my default printer was a printer on my home network. Even though I was not trying to print, Excel and Word were apparently trying very hard to connect to to my network printer.