Slow scan, unresponsive pc

This only started happening recently; when scanning ANY file, large or small, the progress is stuck on 0% for at least a minute or 2, and while this is happening the pc becomes unresponsive, mainly when trying to open programs (task manager doesn’t open at all) and, of course, the hdd light goes mad- most of the time it just stays on. Afterwards, the pc becomes responsive again, and the scan will proceed, but slowly. I re-installed Avast yesterday, which fixed the problem, but it’s doing it again today. I am currently re-installing it again

I’ve tried other free anti-virus programs before, but they weren’t really satisfactory, so I’d really love to keep Avast.

I’ve reached my internet cap, so my download speed becomes limited to around 50 kbytes/s, which can drop quite drastically even if 1 person is on the same network. Maybe this has something to do with it but I’ve had Avast for over a year now so I don’t understand why this would happen all of a sudden.

Any help is appreciated

Please provide more information.

What OS/SP ?
What exact version of avast ?
How exactly did you do the re-installation ?

Windows 7 Professional 64-bit, Service Pack 1
Current Avast version is 10.2.2215
I uninstalled it from Add/Remove Programs, and re-installed it with the “avast_free_antivirus_setup_online.exe” file with a custom installation; installed it in the default directory (C:\Program Files\AVAST Software\Avast), but unchecked some of the extra tools (such as SecureLine and Grime Fighter). I haven’t purchased any tools. The only tools I have is Remote Assistance, Rescue Disk, Browser Cleanup and Home Network Security

Download and save http://files.avast.com/iavs9x/avast_free_antivirus_setup.exe

Download and save http://files.avast.com/iavs9x/avastclear.exe

Run the “avast clear” from above. When prompted to reboot into safe mode click “yes”'.
Reboot.
Install avast from above.
Reboot and register or insert license.