Annoying taskbar buttons

Running XP Pro, over the past few days I have noticed that every few minutes the program buttons in the taskbar start jittering for a fraction of a second. It looks like there is a program that starts and ends really quickly and the button for it never has time to fully appear, but it makes all the other buttons slide around as if they were going to accomodate the new one. Annoying not only in that looks like it is blinking, but I am curious as to what program is doing that. I scanned with virus and spyware scanner, and I am really careful about that stuff anyway so I don't think that is the problem, but I want to make sure. Is there a log or a viewer somewhere where I can look at the execution history or something like that? Thanks in advance!

A little update. I actually discovered that it is not a taskbar button that is appearing, it is an icon in the system tray. I never noticed because I always had my taskbar pretty full so it looked like a new button pushing all the others over, but I saw it one time when I only had a couple windows open. I just happened to be looking down there and saw an icon turn on and off rapidly a few times in the the system tray. It expands the tray which makes the other buttons smaller.

i guess you should check for spyware.