Something is hogging all of your computer's memory and/or processing power.
Open the Task Manager (via right-clicking on the taskbar at the foot of your screen). Click 'View processes from all users' at the bottom left (if that option appears). Maximise the window to fill your screen, click on the 'Processes' tab and then minimise the window down to the taskbar. (You need to have the Task Manger open
before you encounter a problem as you won't be able to open it if something is already hogging all of your computer's resources).
As soon as you encounter a problem, click to maximise the Task Manager window and look for anything abnormal. You're looking for:
(a) a high figure in the CPU column (everything should normally be quite low except for 'System Idle Process' ; or
(b) a high figure in the 'Mem Usage column ; or
(c) a figure which constantly changes every second or two in the 'Mem Usage' column (even if the figures you see are actually quite low).
If you can spot anything there, you might then have an idea of what's causing the problem. In my own experience, anti-virus software continually try to update itself is often the cause. (It's indicated by a frequently changing figure in the 'Mem Usage' column, possibly also associated with a lot of hard drive activity).
I battled for years with similar problems on my old XP machine. (Switching from Microsoft Security Essentials to Avast solved my problems for a while). Eventually though I gave up and forked out £99 for a reconditioned Windows 7 computer from a firm that I've got 100% faith in:
http://www.accomputerwarehouse.com/products/computing/refurbished-base-units
Maybe it's time to upgrade anyway?