Again, I'm not too sure what I'm looking at, but all three shields appear to be running. File is top, at 458, then Web, at 130, then Mail, at zero, I think.
Incidentally, I fiddled about with the settings and turned all reported items ON in the File System Shield report. This produced many lines in the report with c:\...\Dropbox.exe [OK], so I set this file in the Exclusions. This seems to have completely cured the problem (at least on my laptop) - the CPU value now remains at zero when accessing Dropbox (which is a lot faster, now). However, I'm not sure I've done the right thing by including it in the exclusions. Can you confirm that it's OK. And the ticks along side it - do they indicate that Avast will not check them if the file is read, written to, and executed? If so, I assume I only need to have the X ticked, not the R and W, as it's an executable file not one that should normally be changed.
Matt