A few weeks ago I had a problem with WSS. Really strange things happened, but nothing was written to the eventlog. After some searching I found the logging file written by WSS, but the amount of information logged is really minimal. SPS has much more extensive logging, see manual, but WSS specific things are even on an SPS server logged to the WSS logging mechanism.
After some searching I found on http://wss.sharepointtips.com/pages/III%20-%20Administration.aspx on Q: 04 the following text:
"STS 1.0 had logging to C:\WINNT\system32\LogFiles\W3SVC1 (and W3SVC2, W3SVC3 etc.)
HKEYLOCALMACHINE\SOFTWARE\Microsoft\Shared Tools\Web Server Extensions\6.0
Change the LogLevel to Value Data of 9
The Loglevel is by default set to 0. If you look in this container you also find the key MaxLogSizeKB. I assume this specifies the maximum log file size, don't know what happens if this size is reached.
More logging is available after setting this registry key, not overwhelming, but more than it was before! Strange those kind of things are not just documented....