Boot problems might arise at this stage if your PC is trying to start from the wrong device. Launch your BIOS setup program, look for the boot order list and change it so that your hard drive (or preferred boot device) comes first. This will also give the boot process a small speed boost.
It's more likely that one of the boot structures on your hard drive has become corrupted, though, and Windows Vista can usually fix this automatically. Boot from your Windows installation disc, then work your way through the wizard, clicking the 'Repair Your Computer' option when you see it. Choose the operating system to repair, click 'Next', and select 'Startup Repair' to fix the problem. Alternatively, use the tool 'bootrec.exe' to do it for yourself (see www.support.microsoft.com/kb/927392).
XP users have more manual work to do. You'll need to boot from your Windows installation disc, launch the Recovery Console and try the 'FIXMBR' or 'FIXBOOT' commands to fix the Master Boot Record or Boot Sector. However, FIXBOOT is risky if you've got more than one partition, as it may overwrite the current partition table.
The free tool Parted Magic (www.partedmagic.com) may be useful here, as it creates a bootable CD that can fix the MBR and Boot Sector, repair a broken partition table, recover a deleted partition and more. Just be careful. This is a very low-level tool: make a mistake and you could create more issues than you solve.
The Windows XP Volume Boot Sector starts by launching the program 'NTLDR'. You'll see an error if this is missing, but otherwise NTLDR will start and check the root directory for a valid hibernation file ('hiberfil.sys'). If found, NTLDR will restore it to memory so you carry on where you left off.
If you're not waking up a hibernating PC, NTLDR will read the settings held in 'boot.ini' instead, displaying a boot menu when you've got more than one operating system installed. If only XP is installed (or you chose it from the menu), NTLDR will carry on loading Windows.
This process is quite resilient, so if, for example, boot.ini is deleted, the boot can probably continue with default settings (after displaying an error). However, this isn't guaranteed and if NTLDR can't be found then you'll definitely be in trouble. Fortunately there's a very easy way to insure yourself against such problems.
Take a CD, DVD, floppy or spare USB drive, and copy boot.ini, NTLDR and 'Ntdetect.com' across from the root folder of your hard drive. If one of these files becomes corrupted and Windows won't start, boot from this emergency disc instead. Assuming there's no other damage, Windows should start, and you can copy your system files back from the boot device to the hard drive.
If you didn't make any preparations, you can also boot from your Windows CD, launch the Recovery Console and copy NTLDR from the CD to your hard drive (use a command like 'COPY D:\i386\NTLDR C:\', where 'D:' is replaced by the letter for your DVD drive). Or, if a missing boot.ini is the problem, enter the command 'BOOTCFG / REBUILD' to recreate it. (There's more about Recovery Console at www.support.microsoft.com/kb/314058).
Under Windows Vista there is no NTLDR, and instead the Volume Boot Sector locates and launches the Vista Boot Manager, 'bootmgr.exe'. This reads your startup settings from their new location, the Boot Configuration Data file (BCD). You'll then see a boot menu if there's more than one operating system installed.