1] Of the programs I tested, A program resembles the beautiful program I used when I worked with windows 3.1 some years ago but includes many enhancements.

I have downloaded some freeware programs and looked at the way they work. As I don't have any authority to test programs as well as possess no expertise in this field, I named the program as A.

Is my first sentence fine? I used the word 'of' at the beginning. It may be incorrect. I have a hunch it would be fine to use the word 'of' at the beginning in such a context.
Beginning with "of" is all right. Here's another way to write it.

Of all the programs I've tested, [Name-of-program] most resembles a program I used when I worked with Windows 3.1 some years ago. However, [Name-of-program] includes many enhancements.

I prefer CalifJim's version. Non-con your version is also correct but possibly a little over-formal and is making the whole thing a bit complicated.
From the original sentence, I interpret program A as having the enhancements.

My comments are:
Make it clear whether the enhancements belong Program A or the program used in the past.

The 'Of the programs tested' is a pointless phrase unless you include the information that Program A is closest to the other program. Otherwise drop it.

Beautiful does not seem appropriate for a computer program. You can find another word to describe why this program was so good.

My version would be:
Program A resembles the useful/versatile/bug-free/beautifully written? program I used with windows 3.1 some years ago, but includes many enhancements.
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You have used the words ' I have used' at the beginniing. Then you have used the word 'worked and used'. I thought it would be ideal for the sake of grammar to maintain the same tense here.

However, I am not sure about my point.

I have eaten some chicken for lunch yesterday. This is wrong.

It should be the following:

I ate some chicken for lunch yesterday.
If you are comparing the program you've tested with Windows 3.1, then, then, in my opinion, the correct form should be:

Of all the programs I've tested, [name] most resembles Windows 3.1, with which I worked some years ago, although the former includes many enhancements.

Hope not to get too much flack for this one... NON C
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When CJ uses the perfect tense:

"Off all the programs I've tested..."

He is implying "at any given time prior to thr present. When he uses the simple past:

"...a program I used when I worked with Windows 3.1..."

He is associating the action "used" with a definite time implied by "...when I worked..." thus the simple past is the correct form.

I've got my helmet on, just in case....
 nona the brit's reply was promoted to an answer.