DavidR, this should work ok, we have long ago patched all the possibly conflicting entries, such as driver and device names.
Thanks for the clarification.
I have same problem also on Vista (Home Premium SP2, 64-bit) with Avast Internet Security. Have to turn it off to use Firefox. 
Please start your own topic with full details so you can receive help in solving your problem.
It should help to disable HTTPS scanning in the web shield.
Winxp and Vista is dead, get over it. Avast should stop supporting these exploitable systems and focus on stable ones.
You want security? but still choose to install winxp/vista? lol. You are just letting hackers to use your systems to infect others.
BTW windows 10 is lighter than win7. Recommended for high and low end computers. Move on.
Thx Asyn. That helped in my case.
steampunk852: you can buy my new OS if you want. 
How to get FREE Win10
https://www.howtogeek.com/272201/all-the-ways-you-can-still-get-windows-10-for-free/amp/
The reason I did not move from Vista to Win7 was financial too!
I have 2 CAD applications that meant that I would have to buy anew.
Chief Architect = £2000 for a start.
Add in Alibre = £1000
Donations welcome 
They would probably run in compatibility mode in Windows 10.
bob3160
Tried it
There was a registry hack for win7 but every time there was a win update it did a reset.
Also note that one of the big changes in win7 was the addition of a hive.
As a matter of interest I have tested side by side my old dual core Vista against Win10 with a 4core (same clock) Vista is quicker on ‘bare bone’ test.
My Vista runs 34 processes on boot. What does Win10 load?
I’m talking about compatability mode. Nothing else.
For me the only problem about running it in windows 10 compatibly mode, is you have to have installed win10 to test the theory that it may run the programs in compatibility mode and of course the purchase of windows 10. If it doesn’t work you are left with having to get back to your original OS installation.
You’re welcome.