Answer : Several things can cause this to happen. The most obvious possibility would be that the mouse is faulty. The first (and easiest) thing to try is to replace the mouse with a fairly simple one that's known to work on another PC. If that works, then it's a fair bet that the mouse (or the mouse driver) is faulty. Before the mouse is replaced, consider installing the latest version of the mouse's driver software (if one is needed or available). If that doesn't work either, then it's probably time to replace the mouse. But before that, remove the cable and look at it closely. If any pins are bent, straighten them very gently. A pair of "needle-nose" pliers are ideal for this job.

If it's a ball mouse (mechanical or opto-mechanical) try cleaning the mouse ball and the rollers. Mechanical and opto-mechanical mice tend to gather dust and dirt around the rollers and the ball after they've been used for a while - sometimes cleaning them will solve a lot of issues.

Most mechanical and opto-mechanical mice have an "access port" on the "bottom" where the ball protrudes. This "access port" is akin to a manhole cover with a hole in the middle to accommodate the mouse ball.

Most of the time, the mouse's cover can be removed by rotating it counter-clockwise (arrows showing the correct direction of rotation may be embossed onto the cover itself). Rotate the cover to remove it. Care should be taken at this stage because very often, the cover is the only thing supporting the ball -- removing the cover will cause the ball to drop out of the mouse. Because of this, it's best that the mouse be upside down when the cover is removed.

Another reason is because the display driver is bugged. Most modern PCs have something called a "hard-ware cursor" in which the position and display of the cursor is actually controlled by the display card. This "hardware cursor" is used to accelerate cursor rendering and positioning. Because of this, a bug in the display driver can cause the cursor to appear to suddenly go haywire.

A bug in the display driver can lay dormant for a while, only to be triggered when something happens, for instance, if the display card is instructed to put the monitor on standby.

Updating the display driver might be able to solve this problem. Also, to avoid such problems from recurring, the power management options can be turned off. To do this, right-click on any empty area of the desktop, and select "properties" from the pop-up menu.

When the "display properties" window pops up, click on the "Screen Saver" tab, and then on the button that says "Power". This will open a windows titled "Display Properties". From here, click on the downward- pointing arrow just below "Power Schemes" and select "Home/Office Desk".

Next, under "settings for Home/Office Desk Power Scheme", click on the downward-pointing arrow to the right of "Turn off Monitor" and select "Never". Do the same for "Turn off Hard Disks" and "System Standby". After this, click on the button labelled "Save As". A window titled "Save Scheme" will pop up, with "Home/Office Desk" already filled in. All that needs to be done here is to click on the "OK" button. This will save the new scheme.

After the new scheme is saved, click on the "OK" buttons to close the "Power Options Properties" and "Display Properties" windows. This will effectively disable the "power saving" functions in Windows XP and might help solve any problems related to device drivers behaving strangely after the PC idles for a while.

The "power saving mode" can be turned off in the PC's basic input/output system (BIOS). This can usually be done by adjusting a setting in the PC's BIOS firmware. This setting is usually called "Power Management" or a similar term.

One other possibility is that the system's been infected with some sort of malware that makes the cursor go haywire. This can be cleaned up with a virus cleaner software.
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