From http://wiki.vtiger.com/archives

About

Popup reminder will be shown when ever an activity is created at the interval in which the time of activity falls, where the interval is defined by user within the given choices.

Working

Interval

Each user is given 'None' as interval by default.

The interval is given such that the user can configure the time interval in which the popups are meant to be shown for every interval part of time. i.e. By default the popups are not shown for newly created users.

This can be configured to None, 1 Minute, 5 Minutes, 15 Minutes, 30 Minutes, 45 Minutes, 1 Hour and 1 Day

If the interval by user is set to 'None' then popup will be set as not to be shown.

Reminder Popup

Create an Activity and set a time for that activity and save that.

Now At the time of activity to occur, a popup is shown to the user that the activity is assigned to.

The popup shows the status, time of activity, activity subject and options(Postpone or Close).

On Clicking the link Postpone, The activity reminder popup will be shown in the next interval after the click of postpone.

On Using Close click the popup for the activity will be set as marked and will never be shown until the activity is modified for the next time.

Drawbacks

Microsoft IE7 Issue - Reminder Popup's left for more intervals might get blocked and would not dissappear even after clicking on Postpone (until unless page is refreshed), but all the activities with the Reminder Popup's will still continue to work.