Looking for the softwares to monitor our file server

Hi all,
I'm a network manager at smal-size company. We've one file server
using Windows 2003 server. I'm only using Event Viewer to log all
operation with the file server. Recently, some important data in the
file server have been deleted.  So, I can't detect when does this
operation occurs? And Who do this?
To improve the logging capability, i googled and found this software:
Alfa File Monitor (http://www.alfasp.com/products.html). It meets with
our requirement, but very expensive (1.999 EUR).
Do you know any software like Alfa File Monitor (with less price) ?
The software can answer quetions:
- Who does delete, modify the file server?
- When does this occurs?

Many thanks,
--
No Pain No Gain

---------------------------------------------------------------------------
This list is sponsored by: Norwich University

EARN A MASTER OF SCIENCE IN INFORMATION ASSURANCE - ONLINE
The NSA has designated Norwich University a center of Academic Excellence 
in Information Security. Our program offers unparalleled Infosec management 
education and the case study affords you unmatched consulting experience. 
Using interactive e-Learning technology, you can earn this esteemed degree, 
without disrupting your career or home life.

http://www.msia.norwich.edu/secfocus
---------------------------------------------------------------------------