Ouch, too bad about your PC. It seems to happen a lot.
'Necessary' is a strange word, as there are vaccines that you have no matter where you live, and there are those that are not even suggested unless you go somewhere special, like Africa. Probably many of these are not 'necessary', but you don't really know until after the fact, if ever. However, the list isn't as long as it seems, and most are boilerplate.
Yellow Fever: if you visit most subequatorial african countries, you need to show proof of YF vaccine when you get back home. As a result, most African countries look at your YF Vaccine when you arrive, and there are those who won't even let you in without it. Another poster insisted that Kenya does not require proof of YF vaccine, but I can tell you that they read MY vaccination card when I entered about 3 weeks ago. Better to have this document than to try to explain why you don't actually need it as you are standing at the immigration desk, and YF is deadly and ugly enough that you might as well join the 99.9999% of folks here who have the vaccination.
MMR (Measles, Mumps, Rubella): in the US, everyone gets this in childhood, along with Polio and Whooping Cough. You might already have it or not, but if not, its one of those 'childhood innoculations' that has resulted in longer lifespans of most of the first world. As an adult, mumps can kill you, so if you haven't had this, your physician will almost definately just give it to you.
Typhoid: again, the chances of getting typhus are pretty slim as its almost been eradicated....due to the strong vaccination programs worldwide. But if you DO get it (not likely), you'll die. Its another 'childhood vaccine' that is worth having.
Tetanus: Get it. This is one that you should keep updated every 9-10 years, no matter where you live, and getting a cut on your foot or finger is not worth getting tetanus over. This is in the same category as washing your hands before dinner. Just do it.
Hepatitus A and B: these are the only ones that are merely recommended, and not required. If you are staying in nice lodges, eating clean food and doing the 'tourist thing', then your chances of getting Hep are pretty much nil. If you are spending any time in-country, then its greatly recommended to have these onboard, as you can get very sick for life if you catch them.
So, here is the summary: MMR, Tetanus, Typhoid: standard vaccinations that most first-world people have from childhood, and are worth having if you did not get. Add Polio to this list.
Yellow Fever: the only one that they actually ever ask for in Africa, and for a good reason. Look on this one as your Yellow Badge of Passage ('Hey, cool! You had to get a special shot to go to Africa!'
Hepatitus A and B: optional, unless you spend time around open garbage pits. Like most of subsahara Africans do.