This is an old one. I had tried it on Window 2000 as well. Now let me tell you one important fact related with tuning. Value of MTU depends upon how fast a connection you have, what kind of activities you perform (ask yourself a question. Are your a mild, average or high end user of Internet ? To put it in other words, Do you download little or nothing, or are an average surfer or are Internet savvy individual and download everything from programs,apps,patches to watching Internet audio/video real-time content?) Depending upon who you are as a user and what kind of connection (modem,dsl,cable or in rare cases T1/T3), this tuning may have little to significant effect on your Internet experience. All right MTU stand for Maximum Transmission Unit. Value of MTU decides the size of Internet data packet. Bigger the size, more you can transfer. Analogy could be made with a bucket. If bucket is small, you can carry little but it would be no strain on your strength. Bigger the bucket, more you can carry but it strains your physical strength. (Well! Its not that simple but good enough to understand). Objective should be find a perfect balance which works for you. I think maximum that you can put is as value of MTU is 1500 (but could be higher for your specific network) and it may not make sense making it smaller than 68.

Again this is a registry tweak unless and until you have a tweaking utility that can do it for you.Open Registry by going to START-RUN and entering REGEDIT and Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\ ID for Adapter. For this right click on right pane and add a key by the name MTU. For this key you can add a DWORD value from 68 to 1500 (or higher value is certain cases). If you put less than 68, it will default to 68. If you put a value higher than 1500 or your Network permitted max, it will default to permitted max.
The MTU (Maximum Transfer Unit) specifies the maximum transmission unit size of an interface, and is usually determined by negotiation at the link layer of the driver.
The upper level protocols normally optimize pack size for each medium.

Following table gives typical values of MTU in bytes:-

Network Type	Windows XP MTU (bytes)	Windows NT / Windows 2000 MTU (bytes)
16 Mbps Token Ring	17914	17914
4 Mbits/Sec Token Ring	4464	4464
FDDI	4352	4352
Ethernet	1500	1500
IEEE 802.3/802.2	1492	1492
X.25	576	576
PPPoE (WAN Miniport)	1480	N/A