The JET general handbook is probably more comprehensive for information on this, since none of us have had any such problems. Laptops computers should work fine, because modern ones can switch current automatically (but you might want to check if you're planning on buying one before you come), but I don't know about large desktop ones; British ones will need power transformers.
Accessories & Compatibility
Accessories such as printers, scanners and modems can be found fairly cheaply here (for slightly over ¥10,000 each), and shouldn't give you any serious problems. Since the operating software is of course in Japanese, your English system won't be able to read the characters, but this has never caused any problems for me (trial and error works just fine). However you might be lucky enough to buy, like me, a scanner with software written by an American company, and both the original English language version and the Japanese oriented versions are included. However the modem is more problematic. If you buy one here, the Japanese driver software that will be supplied with it might not work with your system. However English versions of these drivers can usually be found and downloaded from the internet. Most computers have a built-in modem driver suitable for all standard types, but with older computers the driver files might only work up to 28Kbps, and you would therefore need a software upgrade to run a modem at the faster speed of 56Kbps. My suggestion is to buy a modem in your home country before coming here (if you can find one cheaply, they sell from around ¥10,000 here), to prevent the possibility of delay once you arrive. ISDN lines can be bought and installed by telephone companies, and a couple of apartments already have existing ones.
Talking of preventing delay once you get here, you'll probably be wanting to get connected to the internet or e-mail once you get here. There are several local companies offering internet access, and we mostly use one called Interlink or American ones (e.g. AOL) with local connection numbers, which has a telehodai ('all-you-can-phone') offer for ¥1050 a month. On top of that you only pay for local telephone calls. Some of these have online application forms, but you might need help with the japanese(?). If you only want to use a computer for e-mail, or you don't own a computer, there are several internet cafes nearby, which are about ¥400 an hour (but printing anything is really expensive).
It's very simple (not to mention completely free) to get your computer fitted with some handy software that will allow it to read and write in Japanese. Microsoft Word (97 onwards) can be adapted to read in Japanese (search for 'japanese support' on Microsoft's site), while the really useful software allows Internet Explorer (version 4 onwards) to read websites in japanese, and also allows its built-in e-mail programme Outlook Express to write and wordprocess in Japanese (search for the 'japanese IME' download on Microsoft's site), which can easily be copied and pasted to other documents. Three of us have also got CD-roms with the necessary stuff on them, which is free (and legal) to copy.
Kanji Gold language study
This is a small programme that can be downloaded free from the internet, or copied from floppy disk (only 1.4MB in size). It's good because it's a lot more interesting than playing Solitaire. It contains around 2000 kanji, starting at the easiest (e.g. ichi, ni), and getting too hard even for a native speaker! You can do it in English (see below) or Japanese. Try it, you might like it...