To change the MTU on your Windows PC using DrTCP, perform the following steps:

surf to http://www.dslreports.com/front/drtcp.html , and click on the most recent version (top of the page) to download it;

use WinZip to extract the file "drtcp.exe" from the zipfile you've just downloaded (your WWW browser may do this automagically; if not, you may need to either run WinZip and "File->Open" navigate to the zipfile, or navigate through the filesystem to the zipfile and double-click);

once drtcp.exe is running, you'll have a picklist of all network interfaces on your computer, with an MTU value probably set to 1400 for each. In each case, change the value to 1300, and click "Save". Ideally, this should allow VPN to work once more.