Athlon 1.4 = 700 Mhz??

i dont if others are suffering from this problem too but here it goes..

specs:
athlon 1.4, ECS k7vza motherboard, 512 RAM, Baracuda HD, Winddows XP

1st boot of my pc in the morning it will run very well. but if i have to restart the pc(say,i just installed a program that needs to reboot the system) it will start really very very slow. i just found why: because when i right click the my computer icon and click on the properties, windows xp reads the speed of the processor as 700 mhz only. if i rebbot again my pc, it will start again very very slow. BUT if i shutdown the computer and leave it turned off for at least 5 minutes, it will get back to its right speed again as soon as it is turned on, difinitely slow if i have just to restart it.

even if with fresh installation of windows xp will not help.still very slow. i have no firewall, no network and this machine is a standalone.

whats going on here? any cure out there? help!!