(If you suspect the problem might be malware, you might want to run a scan with one of these tools.) Whatever the case, you can 'flush' your DNS cache to start from scratch, so your computer looks up web addresses on the DNS server again. Maybe the site you're visiting changed servers, and is no longer located at the cached address, or you have some malware that's trying to redirect common pages to malicious sites. Unfortunately, on rare occasions, this cache can cause problems. This allows your computer to navigate to sites you've already visited, without asking the DNS server every time.
To continue with the previous metaphor, it's like writing down 'Tom's Bistro - 123 Main Street' on a sticky note. In order to speed this process up, your computer saves some of these entries for easy access later on. However, it can take a long time to scan the phone book before you find the right address.
