Note: This has nothing to do with WordPress. This is for the forum software "VBulletin"
You will need access to the MySQL my.cnf configuration file and be able to restart the MySQL server to apply the changes. On a shared hosting account you will most likely not be able to do this unless you have a control panel that allows you to edit the MySQL configuration
1. Make sure Vbulletin Options -> Search Type is set to MySQL full text search. Note: "Search Type" is an extra menu item in the vertical menu on the left side
(see attached image #1)
2. At Vbulletin Options -> Message Searching Options -> Search Index Minimum Word Length put 3 into the text field
(see attached image #2)
Edit May 1, 2009: This step 2. may be not necessary as it relates to the VBulletin default search and not the MySQL fulltext search.
3. Now to the hard part. Edit your MySQL's my.cnf file to add this
[mysqld] ft_min_word_len=3 [myisamchk] ft_min_word_len=3
Save the edited my.cnf and restart the MySQL server.
4. Back in Vbulletin, at Maintenance -> Repair/Optimize Tables, select the "Post" and the "Thread" table and click "Continue". I selected all tables at this stage as it won't hurt to check/optimize them all.
(see attached image #3)
5. At Maintenance -> Update Counter -> Rebuild Search Index, click on "Click Here to do so!" and then on Rebuild Search Index