Why is "no" so hard to say??

Well, it's been a bit of a Friday.
As you remember Wednesday, I gave you a snapshot of my totally full schedule looks like. Well, remember how I told you I had about 4 people who were interested in training but hadn't gotten back to me. Well, I got an email this morning.
"When during the week of the 4th of July can you come down? M, T or Th, F?"
What?
You've GOT to be kidding me! (I've found myself saying this phrase a lot in the last few days...)
Well, no they aren't kidding.
Oh, and they have 1999 vintage software.
Yes, you read that right. Software that is 8+ years old.
I'm afraid to even look at the computer, much less touch it. This is when we'd just moved to a Windows platform and 2 GB was a HUGE hard drive, and we were using two 2GB drives!
There's only one person, well, make that 2 people who would possibly know this version of the software. Neither one of them are me (I started in Sept of 2000). And both of those individuals are swamped.
After talking with my boss, I politely wrote them and told them we'll help them get out their reports, but there wasn't much more we could do for them without upgrading their system.
Why is 'no' such a hard word to say? It took me all day to convince myself that I really didn't have any room in my schedule and that I really couldn't help these people.
Other than that, it hasn't been that bad of a day. Just one of those days where nothing goes quite right.
This weekend should prove interesting. My friend Alan is getting married tomorrow. I'm feeling pretty mixed about it. I really like Al. He's been a good friend to one extent or another for well over 10 years now. I'm not sure he's doing this for the right reasons, but I stand behind him no matter what he decides. He's my friend and I'm happy for him if he's happy.
Well, I'm tired. I did the insomniac thing last night/this morning and it's catching up to me. Peace to all and may you be able to say no when you have to.