Answer: a.

Explanation:
For something that happened a single time in the past, the only correct choice is when. In this context, whenever would mean that I had installed Ubuntu on my old computer multiple times and each time saw a big improvement — which doesn’t really make sense.

When and whenever can both be used to mean at a time that something occurs. They are sometimes interchangeable, but not always.

For example, you could say either “When you go out, could you pick up a newspaper?” or “Whenever you go out, could you pick up a newspaper?”

In the first case, when could refer to a specific time or not. Whenever implies that the speaker doesn’t know when the other person is going out.

However, if you’re referring to a single event that occurs at a specific time, choose when.

Let’s look at another example:

“When I went to the store this morning, I saw Regis and Kelly.”
= You went to the store once this morning and you saw Regis and Kelly there.

“Whenever I went to the store this morning, I saw Regis and Kelly.”

If you use whenever in this sentence, it means that you went to the store multiple times and saw Regis and Kelly each time you were there. The first sentence could mean the same thing — but no one would automatically take that meaning from it. The second sentence can’t mean anything else.

Incidentally, I did install Ubuntu on my old computer and I’m hoping for good things. Running Vista it was brutally slow. I just have to pick up a wireless card and we’ll see how it does.

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Have you got an old computer (or two) hanging around? Want to try Ubuntu? We’ve got some resources you could find useful:

This free chapter download explains Ubuntu installation.

For service providers, this chapter download explains automated Ubuntu installation.

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