WITH an advertising campaign based around the word ''start,'' Windows 95 has done exactly that - and in a big way. One million copies of the Microsoft Corporation software for managing personal computers sold in just the first four days on the market. Buyers included many technology junkies who couldn't wait to try the ''start'' button and other features of the long-awaited software. Now comes the harder part. Microsoft hopes to persuade businesses and average home-computer users across America to upgrade to the new software. The stakes are high not just for Microsoft. If people buy the program, they will often buy new hardware and software programs from other firms at the same time. To keep the cash registers ringing, Microsoft must make Windows 95 appeal to people like Rey Magana. He's a software expert serving 30 branch offices of John L. Scott Real Estate in Seattle. ''Most of our agents and staff are just beginning to catch up to the [old] Windows environment,'' Mr. Magana says. Switching again so quickly could cause confusion, he says. Another reason he's hesitant: Windows 95, like almost any new software product, has bugs that have to be fixed over time. Magana is ''waiting for the report card'' to come out. Problems have already cropped up when computers running Windows 95 try to dial in to a property listings database, says Magana. Should he fret over these bugs if the current system is working OK? For now, he says ''no.'' When the time to move forward does come, he says, ''we may jump into Windows NT,'' Microsoft's high-end platform that will soon get the same easy-to-use look as Windows 95. Similarly, retail giant Nordstrom is mulling over whether to move to the more crash-proof Windows NT or Windows 95 when it upgrades some machines at its corporate headquarters next year, says Larry Shaw, a computer coordinator for the Seattle-based chain. The doubts and questions of business users are echoed by home users. Many of them have machines that don't have enough memory to run the program well. Upgrading could mean $200 or more of hardware costs in addition to $90 for Windows 95. Even Microsoft chief executive William Gates puts the issue in a balanced light in a recent newspaper column: If an upgrade ''costs more than it's worth to you, don't buy it,'' he writes. None of this means the new, highly promoted, software will flop. The program is becoming the standard on new machines. So it will be a hit even if relatively few people voluntarily upgrade. And many users will upgrade to take advantage of specific new features, such as ''multitasking,'' that is, running more than one program at once. The Aug. 24 launch of Windows 95 was aimed largely at home users, says analyst Rob Enderle of Dataquest, a San Jose, Calif., market-research firm. For businesses, he says, ''the first real upgrade opportunity isn't until December,'' when the holiday season gives computer administrators freedom to do the switch.