Xavier,
I too have encountered the same issue with WinHTTrack 
running into the problem.... I haven't paid much attention 
to the logs, but I always restart the download using the 
interrupted download option and run into the same issue.
I have had the problem with URL's like 
(www.astalavista.com, <http://isp-planet.com/>) just to give 
you a couple of examples.  What I have noticed is that the 
problem isn't related to a specific amount of time or 
download amounts... I've had each of the above mentioned 
sites bomb out at various amounts of Bytes saved /links 
scanned.  As far at using time... I have had to pause my 
downloads a few times which added more time than the actual 
download time was.  From what I can tell, it seems to deal 
with the memory and system resouces utilization.  I have 
found my system resources tasked heavily and my ram memory 
seems to drop slowly and stedidly over the time period of 
the download.. Just so you are aware.. I have a PIII 600, 
320Mb ram and well over 80Gb harddrive, Win98Se ( w/latest 
updates from Micro$oft).  I have other programs running ( 
ie: Norton Antivirus, Norton Internet security, FreeMem 
Professional) However, without WinHTTrack running my memory 
seems to stay constant around 220MB Ram Free. With 
WinHTTrack running it drops to 196Mb Free ram at the start, 
then slowly to critical level (setup at 50Mb Ram)  The 
actual error I recieve is <Illegal Page fault> and then 
forces me to close the program.
I will setup another download to get a fresh log, and test 
for specifics, to include time, Bytes saved, Amount of 
links, System resources specs and utilization.  If you have 
any other items or details needed that would be helpful, 
please email me and let me know, so we can get this 
resolved.

Thank you.

PS.  As I was finishing this I had WinHTTrack Bomb out on 
me again with the following error... 

< WINHTTRACK caused an invalid page fault in
module WINHTTRACK.EXE at 018f:0042ade8.
Registers:
EAX=00000000 CS=018f EIP=0042ade8 EFLGS=00010286
EBX=04ddb638 SS=0197 ESP=0166bd68 EBP=0166bdb0
ECX=fffff4bb DS=0197 ESI=0048dcf8 FS=1607
EDX=00000002 ES=0197 EDI=00885000 GS=0000
Bytes at CS:EIP:
f2 ae 8b ca 4f c1 e9 02 f3 a5 8b ca 83 e1 03 f3 
Stack dump:
00010032 04ddb643 0000060d 0166c5cf 04ddb608 008844bc 
572f3a45 61706265 2f736567 20505349 4e414c50 772f5445 
692e7777 7265746e 2e74656e 2f6d6f63 >

Bytes Saved: 71.04 Mb      Links scanned: 1549/23528(+4)
Time: 2h,42min,24s     Files written: 2221