Check your computer for viruses. If your ISP is sure that their connection is okay, then the mostly likely culprit is your own computer. Viruses and worms often use your bandwidth to send and receive data, and if that’s the case, you will certainly observe a drop in your connection speed. Disconnect your computer from the internet and use reliable antivirus software to thoroughly scan your system. Chances are good that your AV will detect something sinister lurking within your computer.
Next, download and install a spyware scanner (my favorite is CCleaner). Scan your system and let the scanner delete everything that it finds. In most cases, the combination of anti-virus and anti-spyware will solve your problem.