Formulating Different Start/Finish Dates with Man Hours

I need help with this and hopefully someone can assist! I am working with a spreadsheet that has different start and finish dates during the fiscal year (Oct - Sept). There are man hours associated with each person and their availability. HOW CAN I FORMULATE TO SHOW THE # OF PEOPLE ASSOCIATED & AVAILABLE AS A RESOURCE WITH EACH UNIQUE TIME PERIOD?????

Example:

Name Start Finish Hrs/Wk 2nd dates hrs/wk
Person A 10/1/07 - 9/30/11 40
Person B 10/1/07 - 8/4/08 40 8/4/09 - 9/30/11 40hrs
Person C 5/30/09 - 9/30/11 40
Person D 2/11/08 - 8/15/09 20 9/10/10 - 9/30/11 20hrs

The dates are in the same column, the only time a start and finish dates are in a secondary column is when the resource had to be taken away from the project and is returning. If you or anyone could help, I could attach the file I am refering to.

All,

Attached is the spreadsheet I am desparately trying to formulate for future use. I have to show "who" is available during the unique time periods for the duration of the project. For example, from 10/1/07 - 2/10/08 there are 23 people available. From 2/11/08-2/15/08 there are 22 people available and so on and so on as you will see on the attached spreadsheet. ANY HELP IS GREATLY APPRECIATED.....

Yes, look at the info on line 33 cell D and E. The time is from 10/1/07 thru 2/10/08 for the number of people available (23.6) and line 34 is the next unique date of 2/11/08 thru 2/15/08 one person (person H) left on 2/10 dropping the total down to 22.6. On 2/16/08 (Person R) leaves at .6 hrs dropping the total down .6 hrs. leaving the total at 22 resources available.