"It doesn't have to be complex to be good" - developer of VCC Windows Virtual Agent
Supports:  
NT 4.0 sp 6+
Windows 2000 Professional, Server, Advanced Server
Windows XP (including support for Emergency Management Services)
 
How do I set up the VCC to run the Windows Virtual Agent? Click here
            
Below is what gets monitored "out of the box": Windows Virtual Agent Tips and Tricks 
Any Windows Event Log:
   Application, Security, System, DNS, etc.
   Variablized time for polling interval gives you total control of when and how often to poll.
   Use SQL like statements (WMI's WQL) to specify the events within the logs (see example below).
   You can specify as many SQL like statements as needed.  

Example:
l_ArraySelectStatements(0) = "select * from Win32_NTLogEvent where SourceName='WSH'"
l_ArraySelectStatements(1) ="select * from Win32_NTLogEvent where EventCode=19002 or EventCode=10 or EventCode=10001"

This example will retrieve event log records whose source name is WSH and records whose event codes are  19002, 10, and  10001. 

CPU Monitor [default values: 90% utilization for 5 minutes]: 
   CPU(s) utilization is monitored 
   If CPU usage is greater than X% for amount of seconds Y, an alert, event and/or action (via script) can take place
   where X and Y are variablized values reflecting a percentage and a time threshold in seconds respectively.
Memory Monitor [default value: 90%+ utilization]:  
   If the following thresholds are exceeded then an event is generated: 
   physical memory usage > X% 
   virtual memory usage > Y% 
   page file usage > Z%
   where X, Y and Z are variablized values reflecting a percentage.
Disk Space Monitor [default value: 95% utilization]:  
   An event is generated if any disk's usage is over X% or the free space is below Y MB
   where X and Y are variablized values reflecting a percentage and a decimal value respectively. 
Service and Process Monitor: 
   All services/processes that are specified are checked to ensure they are running    
   An event is generated if any service is detected as not running, and restarts the service if desired
Example:
l_ProcessArray(0,1) = "MyApplication.exe" : l_ProcessArray(0,2) = "Process"
l_ProcessArray(1,1) = "SNMP.exe" : l_ProcessArray(1,2) = "Service"
l_ProcessArray(2,1) = "sqlservr.exe" : l_ProcessArray(2,2) = "Service"

This example will ensure that process MyApplication and services SNMP and sqlservr are running. 

Other available scripts: 
Host Ping: 
   From the Windows system being monitored, see if it has connectivity to a given host.
Web Server "Ping": 
   From the Windows system being monitored, see if it can get to a given web server.
Database access: 
   From the Windows system being monitored, see if it can access a given data base management system.

System Up Time:
   Monitors the number of hours a system has been operational
   If the system is rebooted during the monitoring interval, an event is generated