Processing User Activity Table

Hello,

I have an application that will be logging to a SQL Server 2000
database user user activity from several Windows 2003 terminal
servers. This information will be retrieved by monitoring the
Security logs of these servers (this part I know how to accomplish
already).

A table in the database, tblLogEntries, will contain the following
fields:
- ID = autoincrementing int
- LogTime = Date/Time the user activity was recorded in the security
log
- Username = User's login ID that the activity was recorded with
- Type = int, referencing a lookup table with the values of Logon,
Logoff, and possible other future items
- Server = The name of the server the activity was recorded on.

The only question I have is, can you offer a way to process the total
user login time during a given range using T-SQL.

For Example...

Given the table data:
ID LogTime Username Type Server
1 10-10-2003 8:30:00 Tom Logon SERVER-A
2 10-10-2003 8:45:00 Sarah Logon SERVER-A
3 10-10-2003 16:45:00 Tom Logoff SERVER-A
4 10-10-2003 17:00:00 Sarah Logoff SERVER-A
5 10-11-2003 8:30:00 Tom Logon SERVER-A
6 10-11-2003 8:45:00 Sarah Logon SERVER-A
7 10-11-2003 16:30:00 Sarah Logoff SERVER-A
8 10-11-2003 17:15:00 Tom Logoff SERVER-A

How would you receive the output:
User Logon Total Time for SERVER-A
Tom 17.0 hrs
Sarah 16.0 hrs

I know I can handle this type of processing on my ASP.NET front-end,
but I'm curious as to how easily it can be done by the database,
itself.

Thanks in advance for your assistance.