it is solved. i searched for this kind of problem in general, and i found a hint to check the temp folder environment variables.
user variables were ok but to my surprise system variables were set to f:\something\win_temp (i think something was temp, so it was f:\temp\win_temp). f: is a dvddrive, so avast could not download to it. i guess avast uses a different method (or user temp variable) to download via the add/remove programs method, so using that i succeeded updating.
for all of those who have the same problem, go to:
start menu/settings/control panel/system/advanced tab/environment variables
and check variables "tmp" and "temp" in both user and system variables. by default they should point to c:\windows\temp (although they might contain other variables like %systemroot%).
and let me add:
THANKS TO EVERYONE WHO TOOK THE TIME AND EFFORT TO HELP US WITH THIS ELUSIVE PROBLEM!
btw in the meantime i ran a few malware detectors none of them found anything.
the "infected" values mbab found are intentional to keep my winxp the way it is (not to show start menu items help and search, force classic control panel etc).
im not sure how i managed to get the system temp variables mixed up, but im happy that its solved, and hope that the others in this topic have the same source of the problem so it can be fixed this easy.
i really missed the few first lines of the log, here is a new one:
Malwarebytes' Anti-Malware 1.50.1.1100
www.malwarebytes.orgDatabase version: 6253
Windows 5.2.3790 Service Pack 2
Internet Explorer 8.0.6001.18702
4/3/2011 1:01:56 PM
mbam-log-2011-04-03 (13-01-51).txt
Scan type: Quick scan
Objects scanned: 152149
Time elapsed: 50 second(s)
Memory Processes Infected: 0
Memory Modules Infected: 0
Registry Keys Infected: 0
Registry Values Infected: 1
Registry Data Items Infected: 4
Folders Infected: 0
Files Infected: 0
Memory Processes Infected:
(No malicious items detected)
Memory Modules Infected:
(No malicious items detected)
Registry Keys Infected:
(No malicious items detected)
Registry Values Infected:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer\ForceClassicControlPanel (Hijack.ControlPanelStyle) -> Value: ForceClassicControlPanel -> No action taken.
Registry Data Items Infected:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Start_ShowHelp (PUM.Hijack.StartMenu) -> Bad: (0) Good: (1) -> No action taken.
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Start_ShowSearch (PUM.Hijack.StartMenu) -> Bad: (0) Good: (1) -> No action taken.
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer\NoSMHelp (PUM.Hijack.Help) -> Bad: (1) Good: (0) -> No action taken.
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer\StartMenuLogoff (PUM.Hijack.StartMenu) -> Bad: (1) Good: (0) -> No action taken.
Folders Infected:
(No malicious items detected)
Files Infected:
(No malicious items detected)