Cleaning out Windows will very likely speed up a PC. And no, I'm not suggesting reinstalling the operating system. There are less drastic fixes.
Your computer may be running a poorly-written process that's hogging a lot of resources. To find out, right-click the taskbar and select Start Task Manager. Click the Processes tab, then the CPU column header. The top items will be the ones hogging the CPU.
If you see an obvious culprit, close the program that's running the process (it's pretty easy to figure out what it is). If that doesn't work, click the End Process button and confirm that you really want to kill this process.
Should that fix the problem, avoid using that program in the future. Or find a better-written replacement.
Otherwise, back on the Process tab, click the Memory column header and see if you can find the culprit there.