I am using windows ® Vista ™ Ultimate on my computer and it is performing very well.
Below is my config according to Everest Ultimate engineer edition....

Processor
CPU Properties:
CPU Type Intel Pentium 4 630, 3800 MHz (15 x 253)
CPU Alias Prescott-2M
CPU Stepping R0
Instruction Set x86, x86-64, MMX, SSE, SSE2, SSE3
Original Clock 3000 MHz
Min / Max CPU Multiplier 12x / 15x
Engineering Sample No
L1 Trace Cache 12K Instructions
L1 Data Cache 16 KB
L2 Cache 2 MB (On-Die, ECC, ATC, Full-Speed)

Multi CPU:
CPU #0 Intel® Pentium® 4 CPU 3.00GHz, 3801 MHz
CPU #1 Intel® Pentium® 4 CPU 3.00GHz, 3802 MHz

CPU Physical Info:
Package Type 775 Contact LGA
Package Size 3.75 cm x 3.75 cm
Transistors 169 million
Process Technology 7M, 90 nm, CMOS, Cu, Low-K Inter-Layer, High-K Gate, Strained Si
Die Size 135 mm2
Core Voltage 1.3 V
I/O Voltage 1.3 V
Typical Power 84 W @ 3.00 GHz
Maximum Power 101 W @ 3.00 GHz

Motherboard and memory
Field Value
Motherboard Properties
Motherboard ID 62-231-0000010-00101111-090506-INTEL$7I6GV231_775I65GV BIOS P2.40
Motherboard Name ASRock 775i65GV

Front Side Bus Properties
Bus Type Intel GTL+
Bus Width 64-bit
Real Clock 253 MHz (QDR)
Effective Clock 1013 MHz
Bandwidth 8107 MB/s

Memory Bus Properties
Bus Type Dual DDR SDRAM
Bus Width 128-bit
DRAM:FSB Ratio 4:5
Real Clock 203 MHz (DDR)
Effective Clock 405 MHz
Bandwidth 6485 MB/s

Chipset Bus Properties
Bus Type Intel Hub Interface
Bus Width 8-bit
Real Clock 84 MHz (QDR)
Effective Clock 338 MHz
Bandwidth 338 MB/s

Motherboard Physical Info
CPU Sockets/Slots 1 LGA775
Expansion Slots 3 PCI, 1 AGI, 1 AMR
RAM Slots 2 DDR DIMM
Integrated Devices Audio, Video, LAN
Form Factor Micro ATX
Motherboard Size 220 mm x 240 mm
Motherboard Chipset i865GV
Extra Features Hybrid Booster

Graphics
Field Value
Graphics Processor Properties
Video Adapter nVIDIA GeForce FX 5200
BIOS Version 4.34.20.87
GPU Code Name NV34
PCI Device 10DE-0322 / 0000-0000
Transistors 47 million
Process Technology 0.15 um
Bus Type AGP 4x
Memory Size 128 MB
GPU Clock 250 MHz (original: 250 MHz)
RAMDAC Clock 350 MHz
Pixel Pipelines 4
TMU Per Pipeline 1
Vertex Shaders 2 (v2.0)
Pixel Shaders 4 (v2.0)
DirectX Hardware Support DirectX v9.0
Pixel Fillrate 1000 MPixel/s
Texel Fillrate 1000 MTexel/s

Memory Bus Properties
Bus Type DDR
Bus Width 128-bit
Real Clock 150 MHz (DDR) (original: 150 MHz)
Effective Clock 300 MHz
Bandwidth 4800 MB/s

nVIDIA ForceWare Clocks
Standard 2D GPU: 250 MHz, Memory: 300 MHz
Low-Power 3D GPU: 250 MHz, Memory: 300 MHz
Performance 3D GPU: 250 MHz, Memory: 300 MHz

HardDisks
HDD1
Disk Device Physical Info
Manufacturer Seagate
Hard Disk Name Barracuda 7200.7 80011
Form Factor 3.5"
Formatted Capacity 80 GB
Disks 1
Recording Surfaces 2
Physical Dimensions 146.56 x 101.85 x 26.1 mm
Max. Weight 544 g
Average Rotational Latency 4.16 ms
Rotational Speed 7200 RPM
Max. Internal Data Rate 683 Mbit/s
Average Seek 8.5 ms
Interface Ultra-ATA/100
Buffer-to-Host Data Rate 100 MB/s
Buffer Size 2 MB
Spin-Up Time 10 sec

HDD2
Disk Device Physical Info
Manufacturer Seagate
Hard Disk Name Barracuda 7200.7 SATA NCQ 80819
Form Factor 3.5"
Formatted Capacity 80 GB
Disks 1
Recording Surfaces 2
Physical Dimensions 147 x 101.6 x 26.1 mm
Max. Weight 635 g
Average Rotational Latency 4.16 ms
Rotational Speed 7200 RPM
Average Seek 8.5 ms
Interface SATA-II
Buffer-to-Host Data Rate 300 MB/s
Buffer Size 8 MB
Spin-Up Time 10 sec

HDD3
Disk Device Physical Info
Manufacturer Seagate
Hard Disk Name Barracuda 7200.7 SATA NCQ 80819
Form Factor 3.5"
Formatted Capacity 80 GB
Disks 1
Recording Surfaces 2
Physical Dimensions 147 x 101.6 x 26.1 mm
Max. Weight 635 g
Average Rotational Latency 4.16 ms
Rotational Speed 7200 RPM
Average Seek 8.5 ms
Interface SATA-II
Buffer-to-Host Data Rate 300 MB/s
Buffer Size 8 MB
Spin-Up Time 10 sec


My vista rating is listed below:

Processor Calculation per second: 4.5
Memory operations per second: 4.5
Graphics Desktop performance for windows aera: 2.4
Gaming Graphics 2.5
Primary HardDisk Data transfer rate: 5.2


What's ur own rating?

Please see attached summary...

Performance Global : 5.0
Performance CPU : 5.0
Performance Memory : 5.6
Performance Graphics : 5.9
Performance Gaming : 5.5
Performance Disk : 5.6


System Summary
Mainboard Intel DG965WH
Chipset Intel P965
Processor Intel Core 2 Duo E6300 @ 1866 MHz
Physical Memory 2048 MB (2 x 1024 DDR2-SDRAM )
Video Card Nvidia Corp GeForce 8600 GT
Hard Disk ST3160811AS (160 GB)
DVD-Rom Drive DVDRW 16X16X ATA Device
DVD-Rom Drive WM2085T QMP222E SCSI CdRom Device
Network Card ADMtek AN983 FastNIC PCI 10/100 Fast Ethernet Adapter
Operating System Windows Vista ™ Ultimate Professional 6.00.6001 Service Pack 1
DirectX Version 10.00
Windows Performance Index 5.0

Via PC Wizard 2008