Is it possible to upgrade laptops?

hard drives can be replaced on almost all laptops.

the rule of thumb as i was told it: if a laptop has a drive larger than 2 gigs, it can use any larger drive.

if it's old enough to not have 2 gigs, then the bios on the machine probably won't accept any

RAM is easy, maybe the HDD. The rest stays put.

Some you can upgrade the processor. It depends on the chipset of the laptop.
I've upgraded my Dell Latitude C840 from a 1.6GHz P4M to a 2.2GHz P4M.

The RAM is able to upgraded, as is the hard drive. However, everything else is part of the system board. If your laptop is a couple years old, then this is definitely the case. Since everything other than the RAM and hard drive is part of the system board, if you want to upgrade then you have to replace the whole board. This is an expensive option, and with laptops as cheap as they are today, I would just buy a new laptop. You can get a brand new laptop with Vista, a dual core processor, and 1GB RAM for as little as $499.