You have had your browser hijacked
You need to take care of that before we can address other things
I have no idea what you are talking about when you say you can type your ip address into a browser window to allow you to change information such as your MTU
You can usually use this tactic to change the information on your ROUTER, but this is irrelevent to your MTU, which is determined by your machine
You can change it by editing your registry or by using any number of utilities, including DRTCP
http://www.dslreports.com/drtcp?r=107
If you had stated originally why you wanted to do this, you would have gotten an anwer much more quickly instead of running around like a buffoon clamoring for help and thinking your issues were more important than everyone elses
BUT LET ME REITERATE ONE LAST TIME--READ THIS VERY CAREFULLY
You are infected with malware, You need to get it cleaned
Visit the malware forum and follow the instructions given. Be PATIENT. Be Polite.