A word of warning if your using Explorer to delete files, do not delete anything from Program Files or WINDOWS. If it's only your own personal data files, no problem. If it's a program, then don't delete it, remove the program using Add and Remove Programs in Control Panel. But under no circumstances delete anything from WINDOWS.
If it's all those junk files that slow down your computer such Temporary Internet Files, History, cookies, recent documents, event logs and so, then why worry, there are plenty of programs out there that will do the job for you automatically. Some are free but even those that are not are quite cheap to buy. For a free version, I've tried CCleaner and it's pretty good at the job. But personally I'm using R-Wipe and Clean, it's a low priced but very efficient cleaner that can be set to do certain tasks automatically at a time to suit yourself. The only problem I found with it was that when you set it to do tasks on shutdown, you will no longer be able to use restart, it would just go to shut down. So I set my tasks to do their job on startup instead. One thing I can tell you is that you will be amazed at the amount of junk it does find.
Even so, that's not all the junk you'll find. There's also a lot more that many people don't realise is taking up huge amounts of space on their hard drives. If you have Windows for instance, it has System Restore Points, and these build up over a period of time taking up a large amount of space.
As long as your not having any problems with the operating system, I would recommend turning System Restore off every so often to clean off all the old restore points. When you turn it back on, you can make a new restore point and begin again.
To turn off System Restore, right click My Computer, go to Properties, click the System Restore tab and uncheck the box next to Turn off System Restore on all drives. Click Apply and then restart the computer. After the computer reboots, don't forget to go back and recheck the box to turn it back on.
To create a new Restore Point, simply go to Help and Support, click on System Restore, then choose create new restore point and follow the simple instructions. Give it any name you like such My Restore Point.
Personally, I prefer to have my System Restore turned off permanently because I have something much better. I have one of the best backup systems there is, Acronis True Image. If anything goes wrong with my operating system, all I need do is restore the image to put everything back to how it was.