I seached google and many people have the same problem...
It's not a problem, and it doesn't need a solution. Just ignore the disk
light flashing and get on with your work.*
Disk activity (at any time, not just after booting) can be due to lots of
things, including:
1/ the indexing service, which makes your searches faster, and is thus a
GOOD THING
2/ superfetch, which speculatively preloads files into RAM that it "expects"
you to need (based on your usage patterns so far), which greatly increases
response times and is thus a GOOD THING
3/ the disk defragger starting up, which prevents your disks from becoming
fragmented, and is thus a GOOD THING
4/ Vista reading the start-up performance data and shuffling the files
around on your hard disk in order to make your system boot more quickly,
which is thus a GOOD THING
5/ your security software (antivirus, anti-malware) updating itself and
scanning some files, which goes without saying is a GOOD THING
In other words, it's just doing what sophisticated OSs of all varieties do.
For all the activities in 1/ to 4/ above, the activity happens at the lowest
CPU priority and lowest I/O priority, and thus backs off immediately you
need the CPU or I/O for yourself.
*I've read that there is a rare and intransigent bug that affects both Vista
and XP, which causes disk thrashing after startup, and significantly affects
performance. Note, this is quite different from the usual Vista
post-startup thrash, which shouldn't affect performance.
If your performance really is being badly affected (and it's not just
psychological due to hearing the disk thrash) you may be suffering from that
bug. I believe the bug has something to do with Microsoft (or Windows?)
Update - you may wish to do some online research if you do have that
problem.
SteveT