How can I reinstall Windows XP without logging into computer?

You do it from a Boot Disc or a Recovery partition. Now as HP has not supplied a Recovery Disc I would suppose you have to press 2 keys when the Boot Screen is up to access the Recovery partition which will do a clean install of Windows so you would have to grab your Owners Manual or download it from the HP Site for your Model or read the screen to see which keys are required to enter the Recovery Partition.

If you no longer have your Owners Manual you can download it here after you enter the correct Model Number

http://www.hp .com/rnd/support/manuals/4108gl.htm

Just remember to remove the space from between the [b]hp[/b] and the [b].com[/b] to get a working link. ;)

Col

Press CTRL+ALT+DEL once when you see login screen, If welcome screen Converted to small login window Do following
At username type administrator (instead of your old account name)
leave password blank . now press enter key It may work if you are lucky (if administrator account not protected by password)

Whilst it is booting try pressing F3(most likely) or F11 to get into the Recovery Console.

Otherwise you will need to provide the model of the laptop for any further assistance.

F9 usually gets you into the boot menu.

Once you know the model number then this site should get you to any other info you need to maybe help you. http://www8.hp.com/us/en/support-drivers.html

You do not have to log in in order for you to reinstall Windows XP.
You need a Windows XP operating system disk.
Insert the disk into your optical drive. Make sure that the disc you use is compatible with with your optical drive and processor (32 bit/ 64 bit).
When you boot your computer it should boot from the disk. If not change your boot order in order so that it boots from the disk.
**Hopefully you have all your system files backed up.
The disk should walk you through the steps of re-installing your Windows XP.