I have an HP All-In-One 2410 and after I had a problem with the scanner part I plugged the USB cable into our laptop and from the wireless router and after that, it kept loading the printer in on start up. I went in to ADD/REMOVE programs and deleted the HP 3.5 version of the printer software. Problem.
What I didn't realize is that it also deleted some of the 4.2 version. After much trial and tribulation I went to the HP site and re-downloaded the 4.2 version.
After downloading this version, and not plugging in the printer when prompted, everything uploaded. When I plugged in the USB cable the printer got recogonized and loaded without fault. The conflict comes when you try to run the provided software and plug in the printer. It appears that XP and the software try to load at the same time and cause the "another installation is trying to load the ---------. Download the latest from HP and the TrayApp message goes away. Also since I took the printer off the wireless router the scanner now works.
I know the answer to your problem of the TrayApp.msi ..
Well the answer is for the TrayApp.msi is located on the CD disc.. Just place in the disc and go inside the cd look for the folder call TrayApp. Inside of that file there you will find what you are looking for.. You might have to repeat it or not.. but it does stop the problem for a good time.. You would do a unastall but it wise if you let it be. because all it does is show you the level of your ink from your deskjet.. I have this problem today [ 09-12-2007] all I did was place in the HP disc deskjet D4200 series into the drive and WALA! there is where it was looking for.. next time just place a copy into the document for the furure .. If any question just ask. I might do some helping out for some answers.. but alot of it comes from the item you buy that have the CDs.