I have a company issued laptop with windows XP installed. I also installed XP on another hard drive using the same serial number for my personal stuff. So when I want to use the laptop for company stuff I put the company one in and when I want to use it for personal stuff I put the personal hard drive in. They are suppose to re-image my company laptop tomorrow, so my "personal infomation" hard drive will no longer be correct in its cmos and such. The machines will be identical, except for serial numbers and such. I do not want to violate any liscensing issues, how do I get the new machines information onto my "personal hard drive" so Microsoft will know that one machine and one operating system.

Thanks in Advance.

By serial Number do you mean Microsoft Windows Activation Key?
I am not sure about the legality of installing the same license number on two hard drives even if you run them in the same machine, you have to run a legal copy in a virtual machine. If you used your employers license to make a separate install of Windows this would to me sound like a pirated copy.
That being said the second drive should continue to work after your employer reimages the original drive and your copy of windows is more than likely as "legal" as it ever was. This is the type of thing that My employer takes a rather dim view of. There are some nice lower priced laptops available now and there are several Linux releases that are quite nice Like Linux Mint and Ubuntu that would cut your risk of running a pirated copy of Windows.