During simulation, Multisim creates a temporary file which is saved to the C:\ drive which contains the actual simulation output. This is the file that Multisim is having difficulty finding. This file path is set by environment variables. If you are in a situation where you do not have write access to your C drive (since you are installed on your G:\) drive I would recommend changing your environment variables with the following procedure:
1.) Start >>Control Panel >> System
2.)Click on the "Advanced" tab
3.) Click on the "Environment Variables" button
4.) Change the TEMP and TMP environment variables to point to a place where you have write access, such as on the G drive.
Although the error I mentioned is for Administrator vs. User access, I would recommend that you try the listed solution, as it will still get you simulating correctly. The problem here is that there is some read/write issue with that particular folder on your computer so changing the environment variables should fix the problem.
I have had just the same problem with NI Multisim 10 Education Edition, Version 10.0.144. The solution described worked fine. I've only had to change the %TEMP% environment variable.
I dare suppose that Multisim does not support national alphabet characters in temporary folder name. In my case Windows user account name consisted of Cyrillic characters. So, creating a different user account (with only English letters in its name) also solves the problem.