If you are starting over more or less from the beginning, I recommend Borland TASM and Turbo Debugger. You can use the Ideal mode and the MASM mode, compare them, and learn even more from running a program through debugger after you assemble.
I doubt if there are distribution problems -- you would have to look at the fine print.
You can get all the old versions of TASM from 1.0 to 5.0 on the net using Google. The cost is nothing or very small amounts. It will meet all your needs for 16 bit programming, and most of what you need to start on 32 bit programming.
Currently most people recommend the A86-D86 website and the HLA website for the latest. They are great. They do cost a little, but are not extravagant at all. But I don't recommend tackling that level until you feel very confident with small programs.
I think these websites are where you can start if you want to look into 64 bit programming. I don't know why people do that. It could be related to the latest in chip developments, but that is very advanced work.
These websites are also very good for online learning - from restart to very advanced.
If you work on a PC, I would not recommend messing with Windows Vista, but earlier versions are OK in either console mode or windows mode.
You could also consider one or another of the many flavors of unix.
Unix has the additional benefit that lots of modern libraries are availble as Free Software - GNU and other sites. Most of this is also ported to windows - certainly everything you need for starting over.
If you prefer books to online learning, try the books by Tom Swan on Turbo Assembler. They are from the 1990's but still very good tutorials for starting over. They are available from out of print sources and are not usually costly.
The best reference I know of up to the year 2000 is by S. Dandamudi on Assembly Language Programming. Certainly covers 32 bit issues and Pentium level chip issues. VERY comprehensive, but not easy to read. Might not be good alone as a learning tool but might do well as a reference along with other books.
There are lots of newer books on ASM programming. Many of them have rave reviews. I just have not had time to read any of them yet.
Hope this helps. It can be fun and instructive programming practice.