Dreaded avrdude "out of sync" ... happy again!

I have been a happy user of Arduino over a year now. Everything started without any problems whatsoever.

But when I switched to another desktop computer with windows XP sp 2 (both had the same os), I could not upload to my Arduino Uno board at all, but got always a message from avrdude "out of sync" or some other abnormal termination (arduino 18, arduino 21, arduino 1.0)

At the same time, my laptop with the same OS worked without problems with the same Uno board.

The troubleshooting instructions are thorough, but during several weeks I could not find any reason for the problem. I was worried: what if my laptop starts getting the same problem? Do I dare to use Arduino at all? Some other people seemed to have the same situation: laptop ok, desktop failing, no reason found.

The verbose output from avrdude both in my desktop and laptop gave the opportunity to compare, where avrdude failed. Originally I was almost sure that the port was not set up correctly, or some other program might interfere by using the same port or something like that.

But with the short "blink" program upload suddenly succeeded once. Next time failed again! The longer the program, the more sure to fail. Additionally, the compiling-linking-uploading process took about 2 or 3 minutes until declaring failure. (laptop about 20-30 seconds). Ahaa, why so slow? Ctrl-alt-del and looking from the sorted list of processes, which programs are hogging cpu time. cmdagent from Comodo took typically over 90 % of cpu time, sometimes 99 %. But I could not kill cmdagent.

Avrdude failed sometimes in the beginning, sometimes in the verification phase, the point of failure varying.

My Comodo firewall free version was from the year 2008 (or so), so I updated to the current free version. Voila! The new version took typically about 20 % of the cpu time, and avrdude executed the uploading of small programs in 20-30 seconds successfully.

Although very few people might use Comodo's old versions, there might be similar problem with other programs, or might it? If cmdagent seizes the cpu too long from avrdude, then avrdude might conclude that no valid response has been got in a reasonable time (timeout? )? Actually out of sync -message is a bit dubious: has avrdude really got an answer from the Arduino board ?

Although I still do not understand the problem fully and the work can continue, I would appreciate your opinions and experiences in similar situations.