My notebook is used in a room which gets nice and warm (30-35C/ about 90F) in summer, and slows down considerably when hot. Steps to rule out various other causes are:
1/ Shut down, wait a minute, start up. Is any software auto-updating? Is a scheduled scan in progress?
2/ run ProcExp - Process Explorer by Sysinternals (now part of Microsoft) to see whether any process is taking a large percentage of CPU capacity; then open the System Information window to get a quick view of System Commit and Physical memory usage, I/O, Network and Disk activity. In ProcExp, hovering over the process name reveals which services are involved, and in the case of Internet Explorer which open tabs use most CPU.
3/ run Speccy - by Piriform, to check on temperatures of CPU, disk and Mobo.
On the RAM page you will find green indicator squares which show at a glance how much memory is free; the CPU page will show core speed and multiplier - is anything maxing things out?; the Network page shows your link speed - is there any internet traffic at all?
4/ run CCleaner : Tools tab to view and disable Startup items; Registry tab to scan for issues; Cleaner to analyze and remove unnecessary bits.
5/ run either Disk defragmenter or Defraggler by Piriform - on XP and Vista I found that it significantly reduced the amount of disk space in use, and reduced loading time; less so on 7, but still worth a look.
6/ even though you may have plenty of physical RAM, Windows will place some content in the swapfile/ System Commit : as I type, 45% of physical memory is in use, and yet a greater amount resides in the System Commit on the hard disk.
As an aside: found that once a hard disk is approx. 2/3 full Windows starts to compress files; both compressing and decompressing take clock cycles and therefore slows things down. Therefore consider
a/ uninstalling all programs which are no longer used (on each update, previous versions are stored on the hard disk, this space is released when the program is uninstalled), and
b/ move archivable files to an external disk/ thumbdrive (again, previous copies of edited files are kept by the OS, and the photo editing software also keeps go-back copies - hence adjusting a photo increases the amount of storage needed threefold, all of which is released once the file is moved off the computer).
And finally, if the computer remains slow : externally store all files you want to keep, de-activate and uninstall all software, then do a clean install; it takes time, but on a Win7 netbook it sped things up considerably. Whether you'll recoup that time by shorter waiting times? Perhaps, perhaps not, but once you're annoyed enough ... .