I downloaded the trial version of Avast Internet Security because I need to review it. The test system didn’t have that program before but the new trial version (supposedly 30 days) says the program has expired on March 19, 2011. Today is March 26, 2011 which is is the day I install the trial version right after the download has finished. I reproduced the issue by restoring my computer to previous "backup image".
Note that the backup image doesn’t have or never had any antivirus program by any vendor because it’s my test system’s image. The only security software on it is the built-in Windows Defender.
Alwil need to check their licensing server for their trial version users soon or you are trying to trick people to buy? I hope it’s not the latter. 
This is what I get the first try:

The second try (after I put restored the system from the backup image — yup, formatted the C: drive before applying the backup image):

I tried clicking update for defs or program hoping that will sync correctly to their server but no joy.
So to anyone trying Avast Internet security program for 30 days and you get the above issue, you’re not alone. Don’t simply buy if you only want to try or test.
BTW, the setup file was downloaded from Avast website which was served by http://www.avast.com/download-thank-you.php?src=http://files.avast.com/iavs5x/setup_ais.exe&product=IS&page=internet-security&locale=en-ww&avast=0 or http://files.avast.com/iavs5x/setup_ais.exe (don’t click unless you want to download the file).
I tried also the setup file from CNET Download.com but it’s same result.
UPDATE – March 27, 9PM GMT +8: After another try (last try), the trial license is working now:

And it is now showing correct date of expiration. I did nothing but to put back again the old system image (backup) just like what I did earlier and install the trial version.