Even if you could restore the system back to a point where you could use the task bar. I would strongly advise against doing that as permanent resolution. New viruses/malicious software are constantly being development and tweak to avoid scanners and make changes that are not yet noticed/discovered/recreated by anti-virus/mal-ware companies. Unless your "customer" has important programs on the system that can not be re-installed/downloaded (ie. he/she lost the discs, the company is no longer in business, etc). With that is mind I would strongly advise you (and your customer) to re-install the operating system and start from fresh. Regardless of being infected your Windows OS is corrupted and will likely not fully-function (...easily). Of course you would do the smart thing and back up to another computer/drive via a slave connection.
Oh and to address your last comment about "its still broke and Avast did it..."
From the sounds of it, you have a "customer" that was/is unfamiliar with Avast and boot scans. Some how that customer managed to start a boot scan and when promoted to delete files, I am guessing that he/she selected the option to delete all regardless of where they were located or what they were. At that point Avast warned him (at least in 4.
that by doing so may delete files needed for system operation (something alone those lines). I am not to sure you can blame someone for something if they warning prior to doing it what may happen.
Anyways good luck and happy computing!