Thank you Ian
I made a test for scanning all files on my PC (2 discs and nearly 700.000 files):operation takes nearly 15 mn.
That want to say any modification (deleting a file for ex or creating another ones,moving files....) is not integrated in the last scan
and any search is not 'live time'.its asynchronous and the search is not fair,the results also in little part.

your idea to update the initial files scan only if modifications occured on system is good.
i remember that in windows events as the sample follow does:

DO FORM home(1)+"samples\solution\europa\windowsevents.scx"

must choose "files folder events" in combo and "all locations" option ("no").Any modification on any file on PC is trapped here but
this recquires the application must run on background all time (service?) to trap all windows messages occured and update the first scan table online
the search result becomes runtime at this step.
a complet scan maybe must be applyed at some long interval (one per week maybe for ex.).

have you experimented this kind of monitoring ?