Windows 10 Install on a "dirty" Windows 7 - system.

If one has a Windows 7 system that has parasites (malware, etc.) and one is going to install Windows 10 while keeping data and apps:

1) It will be more efficient to:
- image the system (in case this was a bad idea)
- install Windows 10
- then look to see how things are.

2) It will be more conservative to:
- clean up Windows 7
- install Windows 10

Any idea if #1 might not only do the install but also clean things up? Any particular steps or methods suggested?

OK. I left out the "make an image" step in #2.

What I don't know is how much is replaced in the upgrade and whether it might not work just as well.
Obviously, it's a risk but there are other considerations.

1. Wipe the drive and install a clean Windows 7.
2. Upgrade to Windows 10.

It seems that I didn't make my objectives clear in asking the question. It's not a matter of "what is best" i.e. *conservative* but what might be the results of doing something a bit riskier.

If all the OS files are replaced then the risk is lower than if they aren't, etc.

But, *of course* I don't want to carry bad things forward. The question is "would this happen?" "Is it more or less likely?" .... Probably too hard to answer.