Old usb devices work fine,new usb devices would not install

Like the subject title, says old usb devices are working fine, but new
ones will not install under Vista. Vista would suggest the update
(Windows6.0-KB940199-x86) but when trying it, I'd get the message that
it does not apply to my machine. I've tried copying the pnf and inf
files, tried moving the usbstore, tried deleting the INFcache.1, all to
no avail (as recommended by many web site). I am currently attempting to
install a Brother DCP-7020 monochrome multi-function laser printer under
Vista via its USB connection (it also has a parallel port but using that
connection, its scanning functions will not work according to Brother's
web site so have to USB to get the scanning ability -- tried an MFC-8040
with the same results and error messages)). I thought of upgrading to
Vista SP1 to see if that would fix things but it's not offered to me
under Windows Update and I know I could install it manually (via the
stand-alone download from Microsoft's site), but am afraid it may mess
up something with my system since Windows Update doesn't offer it to me.
When installing the latest drivers downloaded directly from Brother's
site, after I plug the printer in and turn it according to the
installation's instructions, Vista see it's a DCP-7020,tells me that
it's ready to use, that the USB composite device is ready to use and
that an unknown device failed to install. I even tried the navigating to
C:\Windows\winsx at the appropriate prompt and still nothing. I've even
tried some things with System Restore but System Restore is no longer
successful at any restores for me for some reason, although it doesn't
do me any harm either (it leaves things the way they were before the
System Restore attempt). I have tried the kb941600 download at
http://support.microsoft.com/kb/941600/ which somehow managed to get the
printer side of the dcp-7020 to work but no scanner. In frustration, I
uninstalled that package. I have also thought of re-installing Vista,
but that would be a last resort option as re-installing any OS is no fun
(back up stuff, re-install OS, re-install drivers, re-install programs,
etc.) In case needed, my system's specs are:

Dell Dimension 9200/XPS 410 (same machine but it's known as the 9200 in
Canada and the XPS 410 in the U.S.)
2.4Ghz Quad core
4GB RAM
32-bit Vista Home Premium
Logitech Quickcam Messenger (usb device) -- installed, but not hooked up
- works fine when connected
Seagate FreeAgent Drive 500GB (usb device) -- hooked up and working fine
Brother MFC-665CW (usb device) -- hooked up and works fine
Epson Perfection 1250 (usb device) -- installed, but not hooked up -
works fine when connected
Logitech Meida Keyboard (usb device) -- hooked up and works fine
Logitech LX7 mouse (usb device) -- hooked up and works fine
Sansa E270 mp3 player (usb device) -- hooked up and works fine
LG Lightscribe drive (usb device) -- hooked up and works fine
2 Kingston Data Traveller usb drive -- installed, but not hooked up -
both work fine when connected
IOGear 6 port USB hub with card reader -- hooked up and works fine
DCP-7020 -- trying to install, but so far no go, due to the reasons
stated above.
MFC-8040 -- trying to install, but so far no go, due to the reasons
stated above.

It's getting to the point I am afraid to buy *any* new (to my system)
USB devices as none have worked thus far all of a sudden.

Any help in this matter is greatly appreciated. Thanks.
Chris