What happened to my computer?

My computer was running low on memory and I decided that I couldn't put it off any longer and had to add more. I have done this once before, so after re-reading the instructions on what to do (and what NOT to do), I felt very confident purchasing and adding more. Two nights ago I opened up my computer to reassure myself that I knew what I had, needed and was going to buy. The inside had quite a bit of lint, so I used the hose on my vacuum cleaner and very carefully vacuumed it out. I took the current memory stick out and after looking at it for a brief moment, I put it back in it's place. Then I closed it back up and restarted my computer. Right away, new notices and messages started popping up, including something like, "congratulations on the purchase of your new computer"! What?! Then an option for a tour popped up along with many others to add things and change settings and such. The first thing that I did, was to check for my photos and documents that had been saved. Yay!!! They were still there! Whew!!! What a relief! All of the programs that I had were still showing on my desktop, along with several new ones (ones that I think I deleted shortly after I bought the computer). This seemed extremely strange, but it could be much worse! I went that next day and bought what I needed and added it as soon as I got home. I was so happy to see that it was running MUCH faster again and went to open a program that I use often. It wouldn't open. It said something about the registration info missing. I tried another program that I had added, and the same thing. Then I got a little worried! After trying a few things, I think I am just going to have to add these back one at a time. Has anyone ever experienced this? Know how or why this happened? Maybe have a clue how to easily fix it? :) I asked someone when I was buying the memory, but they told me that they don't understand why this would have happened. I am a pretty optimistic person, and find myself saying a lot, "things could always be worse!". In this case, it is so true! Although I do have an external hard drive that I have saved most photos and important documents to, I could be without that and have lost them on this one as well! Thank you Lord, that it is not as bad as it could have been!