It can totally be avoided .. why do you want to reinstall windows ??
Just
# keep cleaning your registry (there are hell lot of programs for that),
# keep cleaning prefetch (delete all files in the directory)
# keep defragmenting ( obvious suggestion )
Its possible to avoid reinstalling windows all the way !
Reinstalling windows is such a pain .. not the OS itself .. but then you have to reinstall ALL programs ( AV,Firewall,Media Applications, Codecs, etc ,etc) everything. And once you do that .. the system becomes as slow/fast as it earlier was.
Of everything you do .. the above listed things are most imp. .. keep doing them. No need to reinstall.
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alibi