You know you'll have to do it at some point, but surely it can wait...
1 Backing Up
At any point, all the data on your PC could be lost without warning. Mechanical failure, power surges, flood, fire, asteroid strike: any of these things could wipe out years of photos, home videos and documents in seconds (although in the case of the asteroid, you're probably going to be more worried about it wiping out the human race). The answer, of course, is to back up. It's not a complicated idea: you keep extra copies of everything, separate to the ones you access every day. But how many of us actually bother? And even if we do, it's likely we don't do it half as often as we should, and even less probable that we keep our backups in a separate location (unless, of course, we're using cloud storage). So back up your stuff, people, just like we're going to do tomorrow. Or maybe the day after...
2 Clearing Out Old Emails
Anyone who had a GMail account when the service first launched might remember Google's promise that you'd never need to delete another email. Why? Because it gifted you a massive 1GB of free storage. Who could ever fill that, right? Well, as it turns out, pretty much everyone. This promise soon vanished, and the amount of free storage has crept up over the years, but if you're sending and receiving a lot of emails with attachments, you're still going to run out of space eventually. You could pay for an increased amount of storage, but if you don't want to dip your hand in your wallet, then you're going to have to delete some of your old emails. The problem is there are likely to be several thousand messages in your inbox, so it often ends up being a task that doesn't get carried out until you've pretty much completely run of space.
3 Full System Scan
Thanks to the internet, it's never been easier to get viruses and other malware on your computer, so periodically checking to see if you're affected makes perfect sense. But it's also painfully slow and could make your PC unusable while it's going on. So while regular scans are advisable, the truth is most of will instead wait until we're experiencing slow PC performance or other aberrant system behaviour before we do anything. Of course, not all malware makes itself known, so this probably isn't a good idea. A better way of doing things would be to plan a time every week for a full scan, when you won't be using the computer for anything else. The problem, of course, is turning on your PC and resisting the urge to play Football Manager or Fallout 4 all day instead.
4 Sorting Photos
Photos used to be precious things, because film developing could be expensive and you didn't know what you were going to get until you got the prints back. Nowadays, digital photography offers us instant results, and many of us end up with thousands of pictures on our hard drives, taken with dedicated cameras and smartphones. With so many files, it all gets a bit chaotic, so what you need to do is put everything in folders, sorted by year, event or something like that. It'll only take three, maybe four hours. So what are you waiting for? Oh, yes, that's right, it's a job for another time, and actually you don't mind scrolling through 25,000 thumbnails to find that one picture you're looking for.
5 Cleaning Your Keyboard
This perhaps isn't something that many people think they need to do, but they really should. Keyboards get remarkably grubby, with bits of skins, hair and often food getting between and under the keys. You can go years without noticing, but once you do, you'll find it difficult not to see it again every time you want to type something. Naturally, you'll want to do something about this, but you know it will involve taking every key off, running a damp cloth over everything, before putting the keys back where they were. Not hard work, but fiddly, so it's no wonder the most likely outcome is that you'll leave it and simply try to ignore the crunching sound of old biscuits when you press the spacebar.