>>>the hard drive is almost full, will that not slow the computer?
In the 'olden days' (of a decade or so ago!) many computers lacked sufficient RAM to perform complex tasks without needing to temporarily borrow some storage space from the computer's hard drive. So it was possible that a lack of space on thre hard drive could slow things down. These days, most computers have ample RAM and perform the vast majority of tasks without calling upon the hard drive other than very briefly. (Watch the hard driver light on your computer while performing routine tasks. You'll hardly ever see it come on).
If your computer is running slowly there's either something taking all of its processing power or something hogging the RAM. Press Ctrl-Alt-Delete to enter the Task Manager. Click the 'Processes tab' and look down the 'CPU' column. All of the figures (other than 'System Idle Process') should normally be very low. If anything is particularly high, that's the cause of your problem. If 'System Idle Process' is the only high figure, look under 'Mem Usage' to see if there's a figure which is frequently changing; again, that could indicate a problem. (For example, if 'Mem Usage' is constantly changing alongside 'MsMpEng.exe, it indicates a problem with the Microsoft Security Essentials update engine).
Also run a full anti-virus scan and an anti-malware scan with the free version of Malwarebyte's Anti-Malware:
http://www.malwarebytes.org/products/malwarebytes_free/