Computers built for Windows 98 contain hardwares that are by now obsolete. Replacement parts such as memory chips and processor upgrades are harder to come by and they cost a lot more than what you can get for a newer computer. If you merely need a new browser such as FireFox or opera or Adobe flash update, you can simply go to their websites and then download and install them. If you still have dialup internet service, you may want to upgrade it to broadband. AT&T offers high speed internet service for as low as $19.99 a month. Using dialup internet service to download something will take "forever", and it may not work if the file is too large.
I am sorry to say that it is a big mistake to buy a used computer with Win 98 on it. New and refurbished computers are so cheap nowadays that they cost just a little more than a used computer. You can buy a Dell refurbished pc with Windows XP Pro online for just over $100.The best time to buy a new computer is after thanksgiving, which is just barely 2 months away. Package deals may be as low as $300, and new computer as low as $200. After October 22, 2009, all pc sold will have the newest Windows 7 operating system installed.