You already have your system but perhaps you'd like a printer, a scanner or a camera. These are relatively easy upgrades, as you don't need to open the system box. Provided you consider whether you have the correct connections available on your system then at least the new device should be reasonably easy to connect and set-up. But it is still possible to buy an inappropriate product so it still pays to use someone who knows how to best fit a product to your circumstances. eg. if you only want high volume black & white printing then an inkjet would be very expensive due to the on-going printing costs.
Internal upgrades are not so easy and with laptops little can easily be achieved internally (usually memory only or bigger faster disk or smaller but way faster SSD). First of all it's not always easy to decide what you need. Indeed sometimes when your machine has become painfully slow it doesn't need any hardware at all. Even if you do need some hardware, do you really want to poke about inside your PC - I don't imagine you would take the top off your video recorder. Even the simplest upgrade such as memory can be rather complicated as you have to consider which memory slot you have, what is the maximum size of a single unit, what is the required memory speed, can you fit just one unit or do you need matched pairs, do you need to get specially matched memory?
So why upgrade at all. Well by changing key components it's often possible to significantly improve the performance of your machine. Many machines are supplied with insufficient memory. Disk drives invariably end up full and this in turn makes them slow. New video cards can vastly improve games performance. You may be able to get a much faster processor for much less than you might think. Obviously the performance upgrade should be chosen carefully as you can quickly find yourself close to the cost of a new machine, but there is still scope for significant improvement for a fraction of the cost of a new machine.
Perhaps you'd like new features such as the ability to make your own CDs, a modem to connect to the Internet and send e-mails, video editing, DVD reader, surround sound, watch TV (on your PC) or watch your PC on your TV. Most of these are reasonably easy to achieve on most modern machines and some will be fine on older machines. As usual it helps to know what goes with what.
If you don't know what is possible, then perhaps you should ask.
Upgrade your computer with Repair a PC of Uckfield