The always amazing Gibson Research has a free program, DNS Benchmark, which checks your Domain Name Servers then recommends how to optimize them. If you have no idea what that means, the Conclusions tab on DNS Benchmark tells you exactly what to do in understandable terms.
Gibson Research is also the author of Spinrite, the program that has saved hundreds of thousands of apparently dead or dying hard disks (including for me, at least three times.) It is just 180k, written in assembler, and works on PCs, Macs, and Linux. It will revive a dead drive when nothing else can.
Anything from Gibson Research is completely reliable and safe.
Steve Gibson, who wrote both programs, hosts Security Now, a highly useful and informative podcast on iTunes.