OK, I have had 5 months of testing this exact problem ever since i got my new pc.
the specs are
Asus A8n sli deluxe mobo
amd 4400 DC
2 xfx 7800gtx 256 sli
2 gig corsair xms ram
1 western digital sata, 1 seageate Bcudda, 1 samsung spinpoint.
Audigy 2 zs
nec dvdrw
msi cdrw
all chipsets drivers from 6.**
and both cpu drivers 1.2.2.1 and 1.2.2.2
multiple vid card drivers
mobo bios 1011, using 1015 atm
now, the problem lies within the nforce4 chipset or the acpi drivers, Although avast is being locked as a service and isnt unloading.
Because even without avast being installed on my pc, it still hangs on shutdown. less frequently might i add, but with avast being installed it hangs 95% of the time.
I checked event manager and infact there is a load of userenv errors about a unloadable service, the errors only appear after avast has been installed.
I have tried microsofts UPHClean program which supposdly forces unloadable services to shutdown, but that didnt help.
Basically, in 5 months of testing, I have came to the conclusion that its either nf4 (most likely) or my dualcore cpu. As lots of programs, mainly games have problems with dualcore, but out of the 2, I really suspect that the nforce4 chipset is to blame.
Although avast is conflicting for want of a better word with nf4 chipset, as even after a format and just the nf4 drivers and cpu drivers installed, the pc still hangs every now and then, but it does shutdown, sometimes it takes its sweet time in doing so. As soon as avast is brought into the equation the hangs gets worse.
With just windows and chipset drivers and cpu driver, i can shutdown say 30+ times without it locking up, then i may get a few lockups or extremely long shutdowns, then it will return to normal, with avast installed, i can get like 30 lockups/long shutdowns in the space of 40 tries.
I am using xp pro 32
doesnt do it on my other pc.
msi neo mobo
amd64 3000
1gb ram
6800gt
dvdrw-cdrw
VIA CHIPSET