NOTE: Here I'll describe how to change the first tab "General" in System Properties dialog box. Following is a screenshot of the dialog box which will U get after following hack.
1.)
I'm uploading some images, which will be required to be added in dialog
box. Just download them on ur comp and Save them as BMP (Coz the site
automatically converted them in JPG after uploading):
Save it as 1.BMP.
1.JPG
Save it as 4.BMP.
4.JPG
Save it as 5.BMP.
5.JPG
Save it as 6.BMP.
6.JPG
2.) Open %windir%\System32\sysdm.cpl file in Resource hacker.
3.) Now we hv to replace existing 1.BMP file with the downloaded one. To do this:
a.) Goto Bitmap -> 1 -> 1033.
b.) Click on Action -> Replace Bitmap ....
c.) Click on Open file with new bitmap... button.
d.) Select the new downloaded 1.BMP file and click on Open button.
e.) finally click on Replace button.
4.) Now we hv to add remaining downloaded BMPs in the file. To do this:
a.) Click on Action -> Add a new Resource ....
b.) Click on Open file with a new resource... button.
c.) select 4.BMP and click on Open button.
d.) Give 4 in Resource Name and 1033 in Resource Language text boxes.
e.) Finally click on Add Resource button.
5.) Repeat 4th Step for other BMPs (5.BMP & 6.BMP) also. (Resource Name for 5.BMP & 6.BMP will be 5 & 6 respectively and Resource Language will be 1033 for both).
6.) Now goto Dialog -> 101 -> 1033 and replace existing code with the following code:
101 DIALOGEX 32, 10, 376, 215
STYLE DS_FIXEDSYS | DS_MODALFRAME | WS_POPUP | WS_VISIBLE | WS_CAPTION
CAPTION "General"
LANGUAGE LANG_ENGLISH, SUBLANG_ENGLISH_US
FONT 8, "MS SHELL DLG"
{
CONTROL 6, 0, STATIC, SS_BITMAP | SS_CENTERIMAGE | WS_CHILD | WS_VISIBLE | WS_GROUP, 1, 4, 106, 207
CONTROL "Windows Edition", 304, BUTTON, BS_GROUPBOX | WS_CHILD | WS_VISIBLE, 110, 5, 265, 73
CONTROL "", 52, STATIC, SS_LEFT | WS_CHILD | WS_VISIBLE | WS_GROUP, 130, 20, 162, 8
CONTROL "", 54, STATIC, SS_LEFT | WS_CHILD | WS_VISIBLE | WS_GROUP, 130, 36, 100, 8
CONTROL "", 58, EDIT, ES_LEFT | ES_AUTOHSCROLL | ES_READONLY | WS_CHILD | WS_VISIBLE, 147, 192, 107, 8
CONTROL "System", 304, BUTTON, BS_GROUPBOX | WS_CHILD | WS_VISIBLE, 110, 80, 265, 65
CONTROL "", 62, STATIC, SS_BITMAP | SS_CENTERIMAGE | WS_CHILD | WS_VISIBLE, 272, 152, 89, 54
CONTROL 4, 0, STATIC, SS_BITMAP | WS_CHILD | WS_VISIBLE | WS_GROUP, 326, 90, 41, 47
CONTROL 5, 0, STATIC, SS_BITMAP | WS_CHILD | WS_VISIBLE | WS_GROUP, 235, 98, 80, 15
CONTROL "", 63, "Link Window", 0x50000000, 130, 101, 100, 8
CONTROL "", 64, "Link Window", 0x50000000, 130, 91, 100, 8
CONTROL "", 65, "Link Window", 0x50000000, 130, 111, 100, 8
CONTROL "", 66, "Link Window", 0x50000000, 130, 121, 100, 8
CONTROL "", 67, "Link Window", 0x50000000, 130, 131, 100, 8
CONTROL "&More Information", 69, BUTTON, BS_PUSHBUTTON | WS_CHILD | WS_DISABLED | WS_TABSTOP, 235, 123, 80, 14
CONTROL "Windows Activation", 304, BUTTON, BS_GROUPBOX | WS_CHILD | WS_VISIBLE, 110, 147, 265, 60
CONTROL "PID :", 0, STATIC, SS_LEFT | WS_CHILD | WS_VISIBLE | WS_GROUP, 131, 192, 17, 9
CONTROL 1, 0, STATIC, SS_BITMAP | WS_CHILD | WS_VISIBLE | WS_GROUP, 309, 13, 64, 59
CONTROL "This version of Windows is Genuine.", 0, STATIC, SS_LEFT | WS_CHILD | WS_VISIBLE | WS_GROUP, 131, 180, 115, 10
CONTROL "Copyright © 2006 Microsoft Corporation. All rights reserved.",
0, STATIC, SS_LEFT | WS_CHILD | WS_VISIBLE | WS_GROUP, 115, 54, 195, 9
CONTROL "", 56, EDIT, ES_LEFT | ES_AUTOHSCROLL | ES_READONLY | WS_CHILD | WS_VISIBLE, 130, 160, 100, 8
CONTROL "", 57, EDIT, ES_LEFT | ES_AUTOHSCROLL | ES_READONLY | WS_CHILD | WS_VISIBLE, 130, 170, 100, 8
}
7.) Click on Compile Script button.
8.) Now to change the text "Microsoft Windows XP" & "Version 2002" (or something like that):
a.) Goto: String Table -> 12 -> 1033.
b.) Change the String in 180 to "Windows VISTA™ Ultimate Edition" & click on Compile Script button.
c.) Similarly goto: String Table -> 13 -> 1033.
d.) And change the String in 195 to "Hacked by : VISHAL GUPTA" (If u want, U can change "VISHAL GUPTA" with ur name 
) & click on Compile Script button.
9.) At last Save the file and see the magic. 
PS: The image in the screenshot saying "ask for Genuine Microsoft software" is OEMLOGO.BMP which u hv to place at %windir\System32\ folder.
I hv also uploaded my Sysdm.cpl file here.
So just d/l and replace ur file with this one! 
But don't forget to take a backup of ur existing Sysdm.cpl file.
If u face ne problem like version problem, just replace my file with ur default file.