You could possibly set it back to normal in the BIOS if it really has been under-clocked when you sent it in for repairs, NOTE: there may have been a reason for this.
Download CPU-Z (aka CPUID) http://www.cpuid.com/cpuz.php and you will be able to see the details/speed of your CPU
I tested one of our machines at systemrequirementslab.com and it said:
"You Have: 2.80 GHz Performance Rated at 4.20 GHz"
This machine truly has a P4 2.8Ghz, not sure what it means by rated at 4.2 maybe thats what I could over-clock it to
According to CPU-Z
2.8Ghz running at 2.797GHz
Let us know what that CPU-Z says
Though I do like the site, thx for the link.