Download and run AdwCleaner to check for malware (which isn't the same as a virus):
https://www.malwarebytes.com/adwcleaner/
(There's no installation process; you just run the downloaded file).
Then download and install CCleaner:
https://www.ccleaner.com/ccleaner
Run the program and select 'Registry' in the left-hand panel. Click 'Scan for issues' and, when the results appear, click 'Fix selected issues'.
Uninstall any software you no longer use and then run CCleaner, as above, again.
If things don't improve, run a 'repair installation' of Windows. If you need instructions, please tell us the version of Windows you're using and whether you've got the Windows installation disc (or any other kind of 'repair' disc).
If you end up deciding that you need a new computer, please tell us what you'll be doing with it. (i.e. will you just be doing a bit of web-browsing, emailing and word-processing or will you be doing resource-hungry tasks such as playing the latest video games or doing high-end video-editing). We can then point you towards a suitable computer. (Please also let us know whether or not you're in the UK, where this website is based. There's not much point in us directing you to a UK retailer if, say, you're in the USA!)