Help please. Installed avast! about a week ago @ the suggestion of my PC repair guy and have been impressed at everything it found that spybot S&D and McAffee could not find up to that point. Did this because the machine got in the habit of not starting Explorer during start-up and it seemed to fix this problem after removing previous protection applications. But now I have a problem. Did a normal shut-down 2 days ago and when attempted to start machine later that day it gets to the XP welcome screen and stops… no icons, mouse will move, but nothing else happens. Have pressed boot menu (f12) button on start-up many times during repeated attempts to reset but it seems to ignore this. I’m just a user, not a programmer, not sure if this is related to the avast! install but it is the most recent thing to change about the machine. I would really appreciate any help. We are in deep S%$& without this machine working and without access to quicken and on-line bill-pay. Not afraid to get my hands dirty but my experience is limited and will need patient 5th grade level explanations please.
It could be that there are remnants of McAfee left on your system (this could be conflicting drivers), how did you get rid of it ?
McAfee has an uninstall tool that you could run to ensure any possible remnants are removed.
http://download.mcafee.com/products/licensed/cust_support_patches/VSCleanupTool.exe
2007 version - http://download.mcafee.com/products/licensed/cust_support_patches/MCPR.exe
If you can download the relevant uninstall tool (presumably using this system) and transfer by CD/USB, etc. to the problem system. You would probably have to boot into safe mode (avast doesn’t start in safe mode), which hopefully would avoid a driver conflict and run the previously downloaded tools.
Boot into normal mode if this allows you to boot normally great, check that avast is functioning normally.
I can do that but the problem is I can’t even get to a screen or menu that gives me the opportunity to use safe mode. it is absolutely stuck in the welcome screen.
You shouldn’t get to the welcome screen, you should interrupt the boot before it gets there. See booting into safe mode http://www.pchell.com/support/safemode.shtml.
Welcome to the forums, WayneB. 
Instead of pressing the F12 key, try using the F8 key to get to the boot menu where you can select safe boot. Start repeatedly pressing the F8 key just seconds after pressing the Start button on the computer.
http://www.computerhope.com/issues/ch000306.htm
I hope this helps you.
You may also need to press a key on your keyboard in order for the F keys to register.
I use a Logitech Keyboard and I need to press the F lock key quickly after the computer starts to boot
and then press the F8 key in order to get to Safe Mode.