The Moth

With VS2005, if you open the project properties of a VB project and select the "Compile" tab, you'll see a bunch of options.

[digression]Why did the VB team call it the "Compile" tab, whereas the C# team call it the "Build" tab? Although it is a rhetorical question, I'll answer it: Because the two teams do things differently for the sake of it! Consistency between the two is not only not a requirement, but probably looks bad at review time, so they avoid consistency as much as possible. Anyway...[/digression]

So you are on the "Compile" tab and you can choose between the None|Warning|Error notifications for each of the following 9 conditions (we'll see in a moment what the 5-digit numbers are after each one):
1. Implicit Conversion - 41999, 42016
2. Late binding call could fail at run time - 42017,42018,42019,42032,42036
3. Implicit type; call could fail at run time - 42020,42021,42022
4. Use of variable prior to assignment - 42030,42104,42108,42109
5. Function/Operator without return value - 42105,42106,42107
6. Unused local variable - 42024
7. Instance variable accesses shared member - 42025
8. Recursive operator or property access - 41998,42004,42026
9. Duplicate or overlapping catch blocks - 42029,42031

I will not explain each one here, because I hope they are self-explanatory. When you turn On Option Explicit and Strict you'll see that the top 3 are automatically set to Error.

So that leaves the remainder 6. I advise you to set the last 5 to Error; I cannot think of a reason why you would not want to correct issues caught by the compiler - if you have come across a reason, please debate it with me.

So we are now left with Use of variable prior to assignment: Set this to None. The feature is not complete and I have left comments here and submitted the bug here, to no effect. I guess you could turn it on and see if it has caught anything useful and, after wading through a sea of false positives, turn it off again. But this is literally a waste of time.

Finally, let's see what the 5-digit numbers are about. The VB compiler has more than 9 settings; in fact it has at least 24 (represented by the 5 digit numbers). When you make changes to the project properties' tab as discussed above, if you open the vbproj in notepad you'll find a NoWarn tag. Within that tag the series of numbers represents your choices. By enabling one warning at a time, saving and then examining the file, you'll come up with the mapping I have above, e.g.:
<WarningsAsErrors>41998,41999,42004,42016,42017,42018,42019,42020,42021,42022,42024,42025,42026,42029,42031,42032,42036,42105,42106,42107</WarningsAsErrors>
<NoWarn>42030,42104,42108,42109</NoWarn>
The second line instructs the compiler not to bother warning us about possible null reference exceptions. The first line tells it to set all other warnings to errors (this is separate to the "TreatWarningsAsErrors" tag that acts independently on a more generic level).

One question still remains. Why, in some cases, there is more than one number corresponding to a setting? The answer must be because there is finer grained control available that the VB project properties dialog is not offering (in fact, in the C# project properties you can freely enter the corresponding set of C# compiler numbers). Yet another tradeoff between simplicity (i.e. nice UI) and flexibility (i.e. full control).