most probably because of the advanced power management features of vista that manipulate cpu parameters. you mobo might not be able to reset the cpu params properly on a soft restart(like restart from vista)
you can try->
1. update ur bios
2. under vista you can try exploring the options at control panel>click change plan settings under ur selection>click change advanced power settings>scroll to Processor power management and set both minimum and maximum at 100%
3. under bios go to ur cpu paramenters page and choose standard values for everything or try resetting to default settings.
4. update to the latest motherboard drivers for vista
@everyone else - guys comeon. either post relevent stuff or not at all. there are already a overflow of idiot(read useless) bits on the net