Scan does not complete

Hello,
I'm using 7.5 trial on XP with 24 days remaining... If I start a scan it will not complete the scan. I cannot determine when or where it stops. If I leave the scan window on the desktop with the scan active - it will be gone when I come back after a couple of hours? If I try use the tray icon to see what occurred it just disappears when the mouse pointer goes over it If I go to my desktop or start menu icon I can restart it but there is no evidence that it completed the scan... I believe the resident scanner is working correctly. I have uninstalled and reinstalled but this does not change anything. Does anyone have any suggestions?