Nope! You can´t run your old mechanical hard drive as your primary C drive, if you install the OS in the SSD.
Installing the OS on the SDD will make it your C Driver, while your HDD will be your D Driver.
Getting a SSD is the best you can do to improve performance!
It is not just about boot time, but a lot of other things will run smoothly!
The installation is really easy!
Just disconnect the HDD cables (power and SATA). Then connect your SSD, and start your PC with the OS CD on the CD Room, while pressing Delete, or whatever that makes it to get you into the Bios.
In the Bios make your SSD your Primary Boot Device, and set it to AHCI -- you may want to have a loom at your motherboard manual, if you´re not familiar with the Bios features. Then install the OS.
After the installation is completed, turn off your PC, then re-connect the HDD cables, and start your PC -- you´re done!