Yes, by changing the hard drive, you are not violating the EULA.
The EULA (and product activation) allow you to install, change update,
ugrade or transfer your single copy of XP to any system you want, provided
only one copy of the software is installed at a time. Product Activation
won't activate unless you make 6 changes within 120 days, but a quick call
to Microsoft (number will be provided) or reactivation will solve that.
In your case, if you transfer the software to a new hard drive, product
activation won't even be needed. If you reinstall, product activation will
be quick, since you will only be copying your original code.
Courtney