The most common software currently being used for office work is word processing software like Microsoft Word, spread sheet software like Microsoft Excel, and presentation software like Microsoft Power Point. It's important to refresh your skills with these basic programs. Some companies may prefer Apple software and Mac computers, though if you're only expected to do basic computer work you will probably be working with Microsoft programs.
Depending on the field the office is in, they may have specialized software that works specifically for them. You should research what they are using ahead of time if at all possible. For example, you should look closely at the job description and see if it requires you to have experience with any programs. Also, if you have specific experience with a specialized program, be sure to mention it.