The fact that the file is unavailable doesn't mean it is infected, as the scans you have done appear to confirm. But it isn't as simple as that, avast reports the files that it hasn't been able to scan where many other AVs don't report files not scanned, so you would be blissfully ignorant of files that haven't been scanned or for what reason.
There are times when I think avast should do the same as many people automatically think the worst and would want to delete these files that couldn't be scanned. But no it is better to be honest and up front as there are times when these things need further investigation so you need to know.
The winstart.bat file is just a text file (a batch file), which contains commands, these may be to run other processes or tasks when you run it. I have a couple of batch files that I created to back-up me data files to a mirrored location.
I don't have a winstart.bat in my windows folder (using XP Pro SP3), what OS are you using ?
So what we have to find out is a) is this a legitimate file, e.g. have you heard of it/used it before and b) what are the contents of the file.
Exercise Care Not to Run/Execute this file:
To check it, open it with notepad, right click on the file and select open with, from the list choose notepad. This should open it (not run it) in notepad and you can now see the contents.
Copy and paste the contents into your next post.