Restart in Safe Mode. Press F8 shortly after your PC powers up--when it displays the "Starting Windows 9x" text message--to arrive at the Windows 9x Startup Menu. Select Safe Mode, and Windows will load a stripped-down version of itself with a plain-vanilla VGA display driver and a minimum number of other drivers.
Unless Windows is damaged beyond repair, it should successfully launch in Safe Mode. If it does, your problem is likely a corrupted registry file, damaged or missing system files, or another Windows-related problem. Look for the root cause of your trouble by doing the following:
Run Scandisk to look for hard disk damage. A possible cause of missing or corrupted files is a dying hard disk. Run ScanDisk ( Start, Programs, Accessories, System Tools, ScanDisk) or type scandisk at the DOS prompt. Under "Type of test," select Thorough. If ScanDisk finds damaged sectors, that's a sign of significant system corruption or real hard drive problems (modern hard drives map out disk errors at the firmware level, so ScanDisk shouldn't report any). Back up everything immediately. On the other hand, a few cross-linked files or lost clusters is nothing to worry about, especially after a loss of power or a system crash. Just make sure you clean them up, as they can cause problems for Windows.
Look for hardware conflicts. Right-click My Computer, select Properties, and select the Device Manager tab. Scan the listing of your PC's components. If a component is marked with a yellow circle and exclamation point, double-click the component's name. At the center of the General tab is the Device Status box, which will tell you if the component is working properly. If it isn't, click the Resources tab and look in the Conflicting Device list.
If you find a conflict in that list, you must remove it. Do that physically--by removing one of the conflicting devices (of course, by powering down first and following good safety procedures)--or through software, by checking the "Disable in this hardware profile" box at the bottom of that device's General tab in Device Manager. Then try restarting your PC.
Start Windows with step-by-step confirmations. Restart Windows and hold down the F8 key to bring up the Windows 9x Startup Menu. Select Step-by-Step Confirmation. Windows then initializes devices one at a time and prompts you for a "Yes" or "No" before moving to the next device. A "No" skips that device's initialization. A "Yes" should lead to either a message confirming the device's initialization or an error message indicating initialization failure. So if Windows locks up, at the very least you'll know which device was being configured and, at most, you may even find out why.
Check your Registry files. Most of Windows' hardware settings (and most software settings) are stored in the Windows Registry. A damaged Registry file can lead to all kinds of PC problems. Fortunately, Windows 98 provides ScanReg, a utility that automatically creates and stores five days' worth of Registry backups.
To run ScanReg, restart your PC, press F8 to reach the Windows 98 Startup Menu, and select Command Prompt Only. From the DOS prompt you can then type scanreg and restore a backup of your Registry. For more complete instructions on using ScanReg (as well as some more advanced Registry recovery techniques) see "Troubleshoot and Repair Your Registry."
Create a boot log. If you're still stumped, restart once again, return to the Windows 98 Startup Menu (use F8 to get there) and select the "Logged (\BOOTLOG.TXT)" start-up option. As Windows launches, it will meticulously record each step of the start-up process in the file c:\bootlog.txt. Reading the file can reveal exactly what Windows was working on at the moment of its launch failure. The file's long list of techno-gibberish may appear useless, but putting it on a floppy and taking it to a specialist may save you many troubleshooting hours and dollars.