NOTE: AVAST DOES NOT LIKE OTHER AV'S RUNNING ON THE SAME SYSTEM!
...the same time. So two on access Scanners will conflikt with each other and sometimes the driver do, too. So if you want realtimeprotection from avast start MSCONFIG.EXE and unmark all entries belong to NAI/Mcafee. Than restart and your computer should work fine again. You do not need to totaly uninstall Mcafee. A second up to date scanner is allways usefull and the Scanning engine from Mcafee is really good, too.