1. What exact Dell model number do you have?
2. What type of HDD unit do you plan to install? Does you machine have the right connection ports (IDE or SATA) free to use? Do you have the data and power supply cables needed?
3. What HDD do you plan to buy / install?
4. Will this be an additional drive used solely for data storage, or will it replace the original disk completely?
5. What OS are you using, and which version? Certain older versions of Windows need updating before they can use newer large HDD units.