...consider first installing in a VM or sandbox. Verify that everything is OK before installing on your system.
In case it helps anyone, once installed in a VM you have the ability to submit whatever files to virustotal.com, &/or run both portable & installed anti-virus software. FWIW depending on the VM I run the free version of AVG, Microsoft Security Essentials, or Windows Defender.
But is it illegal if I make a portable in a virtual machine without an antivirus and run it a few days later on a real machine?
I'd question the part about "make a portable"... **IF** you mean alter a program itself in any way, &/or reverse engineer a part of it, &/or circumvent its DRM measures, then it might be argued that that was/is illegal use. If OTOH you use something along the lines of Cameyo, which basically records & then duplicates a program's installation in a way that's ideally temporary, you're not doing any of that. You *should* however follow, or at least bear in mind the publisher's intent, e.g. don't distribute a copy of a program you packaged with Camayo or similar.
Otherwise as Chris already posted, the license terms apply to one or more PCs, & a VM is considered a PC. The only time it's not AFAIK is when it comes to Windows itself -- Microsoft sometimes has separate language in its licensing dealing specifically with VMs. To put it another way, most software does not check & therefore cannot tell it is running in a VM, for those just concerned with if &/or how it works. [Note: There are exceptions -- some software & some of the maybe more advanced mal-ware will look to see if it's running in a VM, & turn off.]
As far as installing to a VM 1st, that's what I've been doing for years with most all software, but if you intend to transfer that installation to another copy of Windows, it can get involved, depending on the software you want to transfer. It would be easier to use one or two VMs -- I say 2 because you could run the GOTD, & if you like it run it again in the 2nd, *keeper* VM, & if you didn't like it, restore that initial VM to it's Before state. Either way when/if you want to run that app, run it in the VM.
TO transfer an app from one copy of Windows to another you often need to know what files & registry entries it adds. It could be simple like most GOTD games, where you just copy the program's folder. It could also mean copying several folders & merging a .reg file(s) with hundreds or thousands of registry entries. Sometimes you can install a trial to get many files &/or registry entries into place, then copy just the changes between the trial & the GOTD.
Personally I do it to help avoid sometimes costly errors &/or excessive behaviors because some coder was lazy, inept, arrogant, or in some cases [e.g. the Microsoft C/C++ runtimes] following directions.