TSGL: what makes a computer clock lose time?

[Luminosity]

I have a brand spankin' new computer that my husband and I built in 
January.  The clock keeps losing time.  It's lost seven minutes since 
Thursday.  What could be causing this?  How can I fix it?

I have an ASUS P5K-E/WIFI motherboard and a Intel Core 2 Quad Q6600 
Kentsfield 2.4GHz 2x4MB L2 Cache LGA 775 Quad-Core Processor.

I'm running WinXP Pro.

Thanks,

Luminosity