Doesn't exist. The Intel Pentium 4 is strictly 32-bit. Now if you had an Intel Xeon.....
Just right click "My Computer", go to "Properties" and look there. It will tell you if you have a Pentium 4. If you do, it's 32-bit. It may be a fast 32-bit, but it's still a 32-bit
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The only processor offered to the public in 64-bit are the AMD Athlon 64 and the AMD Opteron (which is basically a server processor). The Intel Pentium 5 will prob. be a 64-bit processor though...