Slow internet on one computer makes the internet slow on others?

[SNikS]

Is this computer older? If so, it's probably using an older WiFi technology. When your WiFi router is communicating with an older technology like 802.11b/g and other devices are using something newer like 802.11n, the router has to perform twice the work. Basically the router is speaking two different languages at a time.

Other things you can try is cleaning up old files and deleting browsing history. A program called Ccleaner helps to do that and can also run registry fixes. Update any wireless card drivers if needed and run a defrag of the hard drive.

And always check for viruses, spyware and malware too.