Tom,
Legacy VPN client is actually end of engineering... otherwise I would advise to send us a copy of minidump generated by windows.
Typically there are two things going wrong:
1) Interaction with some other software on PC (like AV or HIPS software) - try disabling those and checking again.
2) Low level driver problems - try updating your drivers + DNE upgrade - update drivers (if you can) and DNE (Citrix component) and see what happens.
M.