If it's the same immediately after a clean install (back to factory conditions) without being connected to the Internet it's some hardware issue.
But since it's unclear what exactly "a new windows" is (some people only do a repair install while thinking they do a clean install) and since it's unclear what you installed in that new windows (malware?) and run that first 10 minutes (malware, programs with a memory leak?) it can be software also, and that's more likely.
Kees
my computer is very fast on boot and even after startup for about 10 minutes but after that it becomes very slow !
although i had installed new windows several times but it didn't do any thing !
operating system : windows 7 ultimate 32 bit service pack1
RAM : 2 GB
Processor :Intel(R) Pentium(R)3.20 GHZ
Hard Disk : 500.0 GB
Motherboard : Gigabyte Technology Co., Ltd. G41MT-S2PT
Intel(R) G41 Express Chipset

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