Security Suggestions?

Greetings all,
I'm a developer tasked with securing up a SQL Server 2005 SP2 database. I'm not exactly a DBA but I'm giving it my best shot. I was hoping someone could offer some suggestions/tips on how I could approach this task. The amount of documentation on this type of thing is somewhat overwhelming. I'm a little pressed for time and was hoping someone could offer some help. Maybe even provide some feedback as if I'm in the "weeds" or not.

Ok, here's the deal...
At the moment I am using Windows authentication. From what I have read this is the preferred method over SQL authentication. I'd like to continue using this approach if possible.

The database can be has 3 principals
1. ASP.NET (Network Service on Windows Server 2003)
2. Windows Service running on the host server
3. A Data Access Layer assembly running on some other server

All the principals access the db using stored procedures only. Each uses a subset of all the stored procedures, some of them overlap.

My initial though was this:
For the ASP.NET I would perform the following:
1. sp_grantlogin [NT AUTHORITYNETWORK SERVICE]
2. sp_grantdbaccess [NT AUTHORITYNETWORK SERVICE]
3. Grant Execute on [For each sproc used] to [NT AUTHORITYNETWORK SERVICE]

For The Windows Service and the Data Access Layer principal, I was thinking something like this:
1. Create a separate windows login for each principal
2. Create a db login for each principal login From Windows
3. Grant execute on each of the sprocs used for each role

Question: How do I Deny Select, Insert, Update and Delete privs for all tables regardless of the principal (public user)?

Again, any help and or suggestions would greatly be appreciated.
Thank!