As RDA mentioned, ur registry entry might be missing .... in XP there are a number of wayz to get it back ....

1. Click “Start” -> “Run…” (or press Windows key + R)
2. Type “regedit” and click “Ok”.
3. Find the key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Explorer\
Advanced\Folder\Hidden\SHOWALL
4. Look at the “CheckedValue” key… This should be a DWORD key. If it isn’t, delete the key.
5. Create a new key called “CheckedValue” as a DWORD (hexadecimal) with a value of 1.
6. The “Show hidden files & folders” check box should now work normally. Enjoy!

OR

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Explorer\Advanced\Folder\Hidden]
Modify the string value "Type" and enter "group" as value data.
After the edit is complete hit F5 to refresh, and go look in your Folder Options.

Also plz provide more information like OS, version etc along with ur problem.