HELP Guys!!!

I could REALLY use some HELP!!!
Hello folks,

I have an MS Access database where I have info regarding the applications used by the users. The info stored in the database is like-

User - Application - StartTime - EndTime
--------------------------------------------------
John - MS Word - 10:11 AM - 11:50 AM
John - WinAmp - 10:40 AM - 11:16 AM
John - MS Excel - 11:13 AM - 3:31 PM
John - Powerpoint - 2:21 PM - 3:33 PM
John - Gerright - 4:21 PM - 4:51 PM
John - Powerpoint - 5:23 PM - 5:43 PM

I need to find the 'Effective Usage' (Time for which the PC was actually used) of the machine by the user using a query.

Here the 'Effective Usage' would be-
10:11 AM to 3:33 PM, 4:21 PM to 4:51 PM and 5:23 PM to 5:43 PM

I am able to find the 'Effective Usage' in case the applications are executed in series (one by one) But, in real world scenario the applications are executed parallel (some programs being executed while another is running, as shown in example above).

So how do I implement the solution using the existing data?

I know this can be done using good sorting techniques, which unfortunately I am not good at!!! :( :( :(

I would really use some help from you people.

Thanx,

Subhendu