modelsim spawns modelsim which spawns modelsim and so on ... until crash

I have a stubborn show-stopping issue whereby when I open modelsim-Altera-Starter-Edition (with or without using an .mpf project file ) and double click on a .vhd source file to edit it, this spawns another copy of modelsimASE which spawns another etc, etc, etc until the PC crashes.

I've re-installed the Quartus and ModelsimASE several times and also tried several different version of Quartus, but have not yet found the cause or fixed this.

ModelSim ALTERA STARTER EDITION 10.1d

Revision: 2012.11

Date: Nov 2 2012

running on windows 7 ultimate 64 bit , SP1, 16Gbyte ram, fast modern pc.

My colleague has had this issue too, but his "just seemed to stop happening on it's own".

Any ideas on what to try next would be very welcome, as this is rendering modelsim unusable on this pc and I have a lot of vhdl to simulate !!

thanks for your time.