I use the avast 4.8 version on my computer. Up until now I have had no trouble with it. I also have free scanner norton on my computer. It pops up every now and then and runs a scan. Today norton found 2 security problems: w32.spybot.worm and tracking cookie. Not real worried about the tracking cookie. My problem is avast cant find the worm. I have run it thoroughly twice. It came up with nothing. Can someone please help me before my computer dies of infection. I even searched my computer for the file and it couldn’t even find it. Is this one of those false positives that ive seen you talk about? I have just lost my spinning ball in my tray. I tried to turn it off the on in the settings but that did nothing. Please help I think my computer is sick!!! 
Two anti-virus programs on the same computer is a bad idea.
Unistall avast! and reboot as requested.
Uninstall Symantec using this removal tool:
http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
Reinstall avast! and allow the boot time scan.
If you want a second opinion, there are several online scanners you can run:
(Disable avast! while scanning.)
F-Secure
BitDefender
Trend Micro Housecall
ESET Online Scanner