The autocomplete window in IE, mentioned by Bob, is a rather limited context.
In Word, or in Outlook if you use Word as a text editor, this might happen because with the first document Word has to open the spelling files. When this is done, it will keep up.
So where does it happen:
- after booting in MS-DOS
- MS-DOS window
- MS Word
- forms in Internet Explorer
- This answer box
- first screen of the day
- every cell in Excel
The possibilities are endless and the answers might point to a clue.
- Did you thoroughly check for spyware?
- What background processes are running (msconfig, startup tab)?
Hope this helps.