Can't upload anything to Mega 2560

I am new to Arduino and this forum. I had been using my Arduino with the LabVIEW toolkit and on another computer. Recently I bought a new shield and wanted to work with it. Then I discovered I could NOT upload anything to my Mega2560 including the blink example. Here are the details:
Dell laptop running Windows XP. My board is a Mega 2560. The new shield is not connected.
The error I get when I try to upload blink is that Arduino.h cannot be found. Other errors follow but I assume it is because of the initial error
Then I went to an entirelly different computer, also windows XP. Blink uploads fine and runs.
Then I do searches on both the working and nonworking computers for Arduino.h and can’t find it.
I saw a troubleshooting note to make a registry change by adding the key RunOnce. This was already on the nonworking laptop.
I tried reinstalling the IDE, same problem.

Help!

Problem solved. Long story short, what I didn't say in my original post is I was using Sync Toy to synronize my laptop to my desktop computer. I believe this caused the IDE to get confused. I completly removed everything IDE related then did a fresh install. When I started Arduino, it complained the sketch folder was no longer available and the default one would be used.