*Note Didn't read other replies*

1) Apart from desktop programming, I also work on web programming. Does it hurt me to learn PHP and VB.net languages at the same time?

No, it don't do harm in fact it is benifitial, because professionally you may use more web language code than actual VB.NET when creating applications. After pickng up on vb.net I also picked up HTML5 CSS3 JS And minor bits of PHP

2) I think VB.net interface is quite complicated. I did not get used to it after VB6. Where can I find entry level videos or texts where I can learn about menus and all new features?

Wide variety out there including our codebank for VB.NET here on this site, youtube is also FULL of these videos some are entry level pros and some are senior professionals.

3) Visual Studio 2017 does not work fast on my computer. (Usually when ru) Are there any optimization settings? (Operating System: Windows 7 Ultimate 32 Bit, RAM: 3GB DDR3, Display Card: 2GB AMD Radeon, CPU Frequency: 2.67 GHZ)

I don't know.

4) After how long can I start using VB6 in accordance with VB.net and start to improve myself?

I never used VB6, but when I was learning much of the stuff I learned most from was older bits of code, that I later understood how to convert older bits to vb.net, it was pretty simple I thought, because I was mainly focussed on vb.net to begin with. VB6 is nothing to be ahshamed of, so depending on your view, it isn't going to be a "Improvment" aside from going from one version to a newer version. I suggest to try to gradually ditch vb6, as VB.NET is a more modern compelx approach, that offers more extensive libaries etc to develop with.

5) Can I import my old VB6 projects to Visual Studio 2017? Is there a simple way I can do this? Actually I read a lot of article about it, some people say that, "You can import VB6 projects with Visual Studio 2008 Express Edition, after then you can import your new project file to Visual Studio 2017." is it true? (new question)

I think so, but uncertain.