Common ways to end up on spammer lists:

• Posting your e-mail address in public.
• Having an e-mail address that's made up of common words.
• Posting in public places with a username that's identical to the first part of your e-mail address.
• Supplying your e-mail address to companies that sell it to spammers.

For details, see https://en.wikipedia.org/wiki/Email_address_harvesting

If you're flooded with junk mail and your e-mail provider can't manage to filter it, you may want to just get a new address and be more careful in the future. Some providers allow you to set up additional disposable addresses, while most can either download your messages from another provider, or forward your messages to another provider.

• Dealing with junk messages in Thunderbird
• Dealing with junk messages in Gmail
• Dealing with junk messages in Yahoo! Mail
• Dealing with junk messages in Outlook.com

You can also do a malware check with several malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.
All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked