@GazzaGarratt
If you're going to buy a PC Lee, buy an SSD, a good CPU and a decent GPU to begin with and it'll last a long time.
Rib made some good points, particularly on start up. I'm no expert but here's my take:
This summer my PC will be 4 years old. From switching on, to entering my password, to accessing a program/app/internet.... takes about 20 seconds.
No joke.
There has been no slow down in all the time I have had it.
I put this down to (a) buying decent parts (b) not filling it with shit I don't need.
It's main purpose is gaming so I don't have it set up to sync with apps like Dropbox or anything like that and I don't use it to store photos etc.
Like Rib says, an SSD as the drive for the operating system is the only way to go. Good thing about SSDs is that the price is much better than when I bought mine. Also playing multiplayer games from an SSD is highly recommended.
I do have a 2TB HDD for storing most of my games but games like Battlegrounds, Star Citizen, DayZ etc I keep on the SSD.
I've not had to replace anything in 4 years* and - other than a frivolous desire to upgrade the GPU for 4K ultra settings - I don't see any need to replace anything for another 4 years either
Here's my main parts:
CPU: Intel Core i7-4770K (4.3ghz OC)
GPU: MSI Twin FrozR GeForce GTX 770 OC 2048MB GDDR5
RAM: Crucial Ballistix Tactical 16GB
SSD: 250GB Samsung 840 Series
Motherboard: GIGABYTE GA-Z87X-UD3H (Socket 1150
This site is also good for looking at what's recommended at the moment and I used it when purchasing my parts. http://www.logicalincrements.com/
*(other than a new PCI card for the motherboard following a freak lightning strike on my house which fried the ethernet port, cost me about £20. I no longer leave ethernet cables attached to PC or console at night time).