On Windows XP the last McAfee update in last week of August 2013 won't complete as it stops with error saying it can't find a file.
Trying to reinstall from scratch fails also. McAfee check software reckons it fixes but does not.
As McAfee can be turned off by Spyware I use the much more effective WebRoot system as well as Windows Defender.
This all worked quite OK till the last McAfee update.
McAfee full virus scan is UNBELIEVABLY SLOW and takes about 3 DAYS!!!!
If it's the Webroot that includes an antivirus then you are endangering the security of your system because having 2 antivirus applications active will allow infection in eventually. It's fine to have 1 antivirus, plus anti-malware applications such as Windows Defender (except in Windows 8). See the last link in my signature for suggestions under "Free 3rd Party Tools".
If an update failed there may be corruption. Try running the Virtual Techician, a few times if necessary, to see if it will fix anything: http://mvt.mcafee.com/
If not, uninstall McAfee via Control Panel > Add or Remove Programs, then run the MCPR cleanup tool, reboot and reinstall from your online McAfee account.
As far as the scan taking a long time, Technical Support may be the best avenue of approach to that as they can take a direct look to see what is causing it, however there a few things to check first:
Try doing a manual scan rather than a scheduled one and tailor it to exclude whatever rarely changes, external drives for instance if you have a lot of zipped files stored.
Also try turning off the 'use minimal resources' setting - that should speed it up - you will see that setting in Scheduled and Manual scan settings if you scroll down.
A scan can also be held up if it has to negotiate disk-read errors. Try doing a chkdsk /r to correct those. See http://support.microsoft.com/kb/315265/en-us