Can i upgrade a dell latitude xp 4100t

If it's got open RAM slots, you could add RAM. If it supports faster RAM, you could replace the existing RAM with larger capacity ones.

There's not a whole lot of hardware upgrades that are feasible on a laptop.
I concur. Save for RAM and perhaps a new hard drive, i.e. a 7200 rpm drive or even a SSD, your options are limited. Upgrading a CPU in a laptop is pretty hard, especially if you have never opened up a laptop before. Plus, you'd have to ensure that you find a chip that would fit the slot, has the same wattage requirement, and doesn't make too much heat, as a higher performing chip could easily over tax the stock cooling system.

If you are struggling for performance, your best bet would be to upgrade the RAM and SSD and then save for a new laptop in the meantime.