When you first start up your computer, open the Task Manager (via right-clicking on the taskbar at the foot of your screen). Click on the 'Processes' tab. Click on 'Show processes from all users'. (If you're asked for permission to proceed, grant it). Click on the 'Memory' column heading, to place the list in descending order of memory usage. Maximise the window, so that it fills your whole screen and then minimise it down to the taskbar. (It's important to do that before things start going wrong, as you might experience difficulties getting into the Task Manager when they do).
As soon as your computer starts slowing down, maximise the Task Manager window again. Look down the CPU column for any high figures. (Under most circumstances the only 'double figure' entry should be against 'System Idle Process', which might typically have a figure of around 90 or more next to it. Everything else should normally show '00', '01' or '02'). Note the name of any process with a high CPU figure.
Then look down the 'Mem Usage' column. Anything above about 30,000 (other, possibly, than your web browser) should be regarded with suspicion. Again, note down the name of any suspect process.
(Note: If svchost.exe turns out to be causing a problem, look to see what's making a call upon it. You can usually identify what it is by looking for a figure in the 'Mem usage' column that changes every second or two; the actual figure shown doesn't necessarily have to be that high. Again, note the name of the process).
Once you've identified a suspect process try uninstalling the relevant software, restarting your computer and then (if required) reinstalling the software. If you don't recognise a process name, google it (or ask here) to find out what it is. For example, if Navw32.exe was exhibiting unusual behaviour, you could quickly find out that it's part of Norton, leading you to uninstall Norton and then reinstall it. (NB: If it actually is Norton that's causing you problems, make sure that you uninstall it with the Norton Removal Tool, not from your list of installed programs).