Upgrading a processor

I have a friend that is interested in upgrading some components in his PC. One of those is the processor. He currently has a Celeron chip in his mobo. His mobo will support a P4 so he wants to upgrade that. My question has to do with the operating system and applications. If he upgrades his processor will he have to reinstall everything from scratch? I'm thinking this will be the case, but I'm not certain. Let me know. Thanks.

Nope. Upgrading the mobo can and will cause grief but not the processor.

You'll most likely get away with upgrading a Celery to a P4 without Windows complaining. At worst, all you need to do is an over-the-top reinstall, so you won't lose your apps and settings.

Just make sure that the mobo/BIOS supports the Intel P4 processor in question.