Since the OP has asked for a solution that will work on windows, reformat.sh from BBMap can be used like so (NN = number of characters you want on one line)

reformat.sh in=your_file.fa out=new_file.fa fastawrap=NN

On windows you would use it like so (adjust windows file path for BBMap install)

java -ea -Xmx200m cp=C:\bbmap\current\ jgi.ReformatReads in=x.fa out=y.fa fastawrap=NN

from this

https://github.com/jimhester/fasta_utilities

you can use wrap.pl "limits FASTA lines to 80 characters"

Or you can use from here

http://hannonlab.cshl.edu/fastx_toolkit/commandline.html#fastx_clipper_usage

fasta_formatter

fasta_formatter -i yourfile.fasta -w 80 -o yourout.fasta

[-w N] = max. sequence line width for output FASTA file. When ZERO (the default), sequence lines will NOT be wrapped - all nucleotides of each sequences will appear on a single line (good for scripting).

advice: If you will work in bioinformatics try any Linux flavor

Here's a quick one in python:

import sys

inputfile = sys.argv[1]
length = int(sys.argv[2])

print '\nConverting ' + inputfile + ' to ' + str(length) + ' chars per line...'

with open(inputfile, 'r') as f:

     h = []                                             #headers list
     s = []                                             #sequence list
     for line in f:
           if line.startswith(">"):                      #grab headers
               h.append(line.strip().split(">")[1])
          else:
               s.append(line.strip())                   #grab sequences

myseqs = dict(zip(h,s))                                 #map to dictionary, header:sequence

outfile = open(inputfile.split(".fasta")[0] + '_multi-line.fasta', 'w') #open outfile for writing

def print_seqs(header, sequence):
        print >> outfile, '>' + header
        while len(sequence) > 0:
                print >> outfile, sequence[:length]
                sequence = sequence[length:]

for i in myseqs:
        print_seqs(i, myseqs[i])
print 'Converted ' + str(len(myseqs)) + ' sequences.'
  print 'Done.\n'

outfile.close()

Run as 'python your_input_fasta Nchars'

EDIT

A cleaner, more memory efficient solution, per WouterDeCoster comment:

import sys

inputfile = sys.argv[1]
length = int(sys.argv[2])

outfile = open(inputfile.split(".fasta")[0] + '_multi-line.fasta', 'w') #open outfile for writing

with open(inputfile, 'r') as f:
     for line in f:
        if line.startswith(">"):
                print >> outfile, line.strip()
        else:
                sequence = line.strip()
                while len(sequence) > 0:
                        print >>outfile, sequence[:length]
                        sequence = sequence[length:]

if your data can fit in memory, here is a simple HTML form that should do the job:

seqmagick:

library(seqmagick)
fa_read(INFILE) %>% fa_write(OUTFILE, type = "interleaved")