How do you know it's not just a problem with the software you're using to check the CPU? You can also use Intels own CPU identification tool and CPUid (Google for it) for a 2nd/3rd opinion. More to the point, if you are worried your PC seems to be slowing down and you don't know the cause, it's extremely unlikely that it's due to the CPU suddenly being on a go slow. It's more likely that your hard disk drive has become cluttered and fragmented and that programs which have been installed over time are now taxing the system in ways it didn't originally have to cope with. A system recovery (using a recovery CDROM or hidden partion) or otherwise a fresh reinstall of windows and your software will usually address these things and get your system working again at its original speed. Be aware that you should back up your valuable data that you don't want to loose before performing either of these procedures.
Never heard of "PIU test". So you would change to a CPU with a slower FSB speed to match your mother board? That won't increase your performance. Considering how old your CPU is, and how old your mother board must be, you should at least get a mother board to match your CPU's FSB speed, that would likely be cheaper.
I wonder if old parts like Pentium 4 CPUs are sold in other countries where the economies are not so robust.
No, it not caused by OS.
I find it caused by CPU FSB. My CPU FSB is 800MHz, but mainboard just support 400MHz&533MHz.
Maybe i need change another cpu.
Is this a system that's been in use for some time (as is) or have you recently put it together. What does Intels own processor ID tool say you have installed? The intel site doesn't mention C rev 2.80 GHz P4 CPUs (not that I've seen) but A, D and E do feature. How do you know it's a C rev and that it has an 800 MHz FSB? Have you gone into the boards BIOS and selected 'Load Optimum Defaults'?