It is a real good practice that if you want help to provide at least some details.
Telling a mechanic “there is something wrong with my car” is not really helpful.
What OS/SP ?
What exact version of avast ?
Any other security (related) software installed ? (or was there)
Already performed a repair of avast ?
Already did a clean installation of the latest avast ?
Unable to start scan
The IOCTL call made by the application program is not correct.
Will not start any scan. Using Windows 7 x64 with Avast Internet Security ver 2245.
No other security software
Tried program repair thru control panel. No change
Did clean install. No change
Whatever that ^ fix it program is, and I expect in this case it is recommended from personal experience and use, if I had come across it anywhere else I would have been suspicious.
It does not look like an official MS tool and the name of the site, suggesting that it might be, immediately raises concern.
Is it anything more than another of those “system optimisers”? At best they do little more than run CHKDSK, defrag, file clean, maybe an AV/AM scan included too and sometimes driver update. At worst they’re little better than malware themselves installing who knows what and stuff up your sytem.