Like the rest of the responses without information it is impossible to give a specific answer.
I just fixed a computer with the same complaint, very slow. It had a "aquarium" screen saver with swimming fish, a live news banner, live weather, 5 tool bars on I.E., pop-ups searching for ads and coupons on very key push, and nearly every app on the computer started at boot up. Many of these go out on the web at start-up to look for up-dates and to report your surfing history, all while you wait. My customer called the next day and said "its great, it is much faster, but how do I get all of my stuff back??"
The big 4 speed improvements are as follows:
1.) Minimize vampires by killing unused/hidden apps and Ad ware, a OS re-install is a good way to do this. Also Norton or McAfee virus packages can add a lot of overhead. I use MS MSE, it's there but I rarely know it.
2.) Plenty of unused disk space on a defragmented drive
3.) Adequate memory (>3GB of 32 bit OS and >6GB for 64bit OS)
4.) A separate video card (eliminates delay of video accessing system memory and frees system memory for the system)
I have several computers ranging from:
an old Pentium D dual core running XP (32bit)
Various i5 and i7 PCs with Windows 7 64
One with 2 Xeons (6 cores ea with HT) for 24 "cores" total and 128GB of memory running Ubuntu.
While the Windows-7 machines boot 30sec faster than the old Pentium D / XP machine, this old machine is as quick opening apps like MS Office or for web searching once up. All due to following the 4 rules above, especially #1.
The Xeon/Ubuntu machine takes many minutes to come up, sounds like a turbo leaf blower at first, and is fairly slow at general tasks. It shines at complex simulations using custom multi-threaded apps.