i have a problem on my other laptop(this is other than my own laptop about which my other thread's i have post and so i dont want the problem of this post to be related to my previous post's) that is pendrive is not detected. when i see in the device manager on the USB mass storage device, there is ! sign which indicates it's having a problem. and then double clicking on it, there is written that :-
A driver (service) for this device has been disabled. An alternate driver may be providing this functionality. (Code 32)
but the driver USBSTOR.SYS is there on my laptop.
other than this sometimes, pendrive is detected but most of times it's not detected.

Reformat the pen drive, and cross your fingers. It appears to have a typical corruption.

Reformatting the pen drive may make it useful again but you'll loose all current data on it! If that is OK, certainly do try the reformat first.

Otherwise,
To start: see USB Device Problems? Check for Windows Updates

Then, sounds to me, you need remove your upper/lower disk class filters (as one of them may be interfering and the cause of the driver error See for instructions on how to remove your upper and lower class disk drive filters

Then report back

We have not seen that solution work, and have not found it to be helpful in the 100's of usb flash drives that have been brought in for repair...
We would like to hear from people on this forum who have found it a helpful or useful approach to fixing the darn things.

Understand you wouldn't see the problem with filters by looking at a usb flash drive itself (Filters are an issue with a user's Windows installation)

Windows 2000, XP and Vista provide a "layered" approach to drivers. You should think of filters as a special type of Windows driver (which in fact they are).
>> I'm sure you're familiar with seeing a bad driver preventing access to a device.? (e.g. disks or CD/DVDs)
>> Same applies to corrupt and/or bad filters which can also prevent access to a disk (or other types) of devices. Filters are supposed to work with a device's main driver (which is called the function driver). But filters can and, in fact, may be the reason the why the main driver (i.e. the function driver) doesn't work at all

Understanding Windows drivers (and filters) requires a better understanding of Windows internals. To help explain things, see this technical post i've been working on (is still a draft). See So what the heck is a Windows Filter Driver?(And why you should care to know)
While this technical post is still a draft (i have yet to complete) i think it will provide much information. Just let me know if you still have any questions about Windows filters and i'll try to explain as best i can
(you can also search Microsoft's Technet and MSDN (Microsoft Developer Network) for more technical reference and/or more article/detail... or, if needed, let me know.. i can provide you a few of the technical links as well) Or, again, ask. and i'll be happy to explain

/* EDIT */
Oh. and just a quick simple reference to a practical situation where filters have screwed things up here's just one (of many of Microsoft's Windows KBs and FixIts) that instruct users to remove (and fix) their upper and lower class filters (read my tech post and one can now understand why this fix can help!) See Microsoft Fix it Solution to delete lower & upper filters for USB or external hard disk drive

only your pc doesn't recognize it?