First you should check the hardware and software for compatability. you can do this by using the following switch: ie: d:\i386\winnt32 /checkupgradeonly from a command line (assuming D: is you cd drive)
Checks your computer for upgrade compatibility with Windows_2000. For Windows_95 or Windows_98 upgrades, Setup creates a report named Upgrade.txt in the Windows installation folder. For Windows_NT 3.51 or 4.0 upgrades, it saves the report to the Winnt32.log in the installation folder.
And also ensure the data is backed up before doing the upgrade just in case.